Abdullah ibn abi Sarh, the foster brother of Uthman Affan, was a writer of the Quran.  According to some sources, he left Islam because he was able to influence the revelation with his own words and he doubted Muhammad was receiving revelation from God.  The first source which is the seerah of Ibn Hisham says that he was a companion who apostated and Prophet Muhammad wanted him executed.  However Uthman interceded for him and requested immunity, which was granted.  However according to our sources, Prophet Muhammad still wished that he would have been killed and questioned his companion as to why he didn’t kill him. This was also mentioned in Tabaqat al Kabir by Ibn Saad

The story of Abi Sarh being able to predict the next phrase of the Quran is in Tafisr al-Baidawi.  I don’t know how authentic these stories are.  However this is very similar to the just in time revelations issue and I would not be surprised if it was true.  At the very least we know that Muhammad wanted him killed because he used to receive revelation and he was a major threat to Islam.

Baidawi (died 1286) writes:

“‘To me it has been revealed’, when naught has been revealed to him” refers to `Abdallah Ibn Sa`d Ibn Abi Sarh, who used to write for God’s messenger. The verse (23:12) that says, “We created man of an extraction of clay” was revealed, and when Muhammad reached the part that says, “… thereafter We produced him as another creature (23:14), `Abdallah said, “So blessed be God the fairest of creators!” in amazement at the details of man’s creation. The prophet said, “Write it down; for thus it has been revealed.” `Abdallah doubted and said, “If Muhammad is truthful then I receive the revelation as much as he does, and if he is a liar, what I said is a good as what he said.” (Tafsir Anwar al-Tanzil wa Asrar al-Ta’wil [by Abdallah ibn Umar Al-Baidawi])

Ibn Al Athir (1160-1233 AD) writes:

He converted to Islam before the conquest of Mecca and immigrated to the Prophet(P) [i.e. in Medina]. He used to record the revelation for the Prophet(P) before he apostatized and went back to Mecca. Then he told Quraysh: ‘I used to orient Muhammad wherever I willed, he dictated to me “All-Powerful All-Wise” and I suggest “All Knowing All-Wise” so he would say: “Yes, it is all the same.” (Ibn al-Athîr, Usûd Ulghâbah fî Ma’rifat Is-Sahâbah, 1995, Dâr al-Fikr, Beruit (Lebanon), Volume 3, p. 154)[ref]Abdullah ibn Abi Sarh: Where is the truth?[/ref]

In the hadith we hear about how Muhammad wanted him killed:

It was narrated from Mus’ab bin Sa’d that his father said:

“On the day of the Conquest of Makkah, the Messenger of Allah [SAW] granted amnesty to the people, except four men and two women. He said: ‘Kill them, even if you find them clinging to the covers of Ka’bah.’ (They were) ‘Ikrimah bin Abi Jahl, ‘Abdullah bin Khatal, Miqyas bin Subabah and ‘Abdullah bin Sa’d bin Abi As-Sarh. ‘Abdullah bin Khatl was caught while he was clinging to the covers of Ka’bah. Sa’eed bin Huraith and ‘Ammar bin Yasir both rushed toward him, but Sa’eed, who was the younger of the two, got there before ‘Ammar, and he killed him. Miqyas bin Subabah was caught by the people in the marketplace, and they killed him. ‘Ikrimah traveled by sea, and he was caught in a storm. The crew of the ship said: ‘Turn sincerely toward Allah, for your (false) gods cannot help you at all in this situation.’ ‘Ikrimah said: ‘By Allah, if nothing came to save me at sea except sincerity toward Allah then nothing else will save me on land. O Allah, I promise You that if You save me from this predicament I will go to Muhammad [SAW] and put my hand in his, and I am sure that I will find him generous and forgiving.’ So he came, and accepted Islam. ‘Abdullah (bin Sa’d) bin Abi Sarh hid in the house of ‘Uthman bin ‘Affan, and when the Messenger of Allah [SAW] called the people to give their Oath of Allegiance, he brought him, and made him stand before the Prophet [SAW]. He (‘Uthman) said: ‘O Messenger of Allah! Accept the allegiance of ‘Abdullah.’ He raised his head and looked at him three times, refusing his allegiance each time, then he accepted his allegiance after three times.

Then he turned to his Companions and said: ‘Was there not any sensible man among you who would get up when he saw me refusing to give him my hand and kill him?’

They said: ‘We did not know, O Messenger of Allah, what was in your heart. Why did you not gesture to us with your eyes?’ He said: ‘It is not befitting for a Prophet that his eyes be deceitful.'”  (Nasai)

In Alfiyat al-Sirat An-Nabawiyyah by Al-Hafiz Zain al-Din ‘Abd al-Rahim al-‘Iraqi (1325-1404 AD) he writes:

“The scribes of Muhammad were 42 in number. `Abdallah Ibn Sarh al-`Amiri was one of them, and he was the first Quraishite among those who wrote in Mecca before he turned away from Islam. He started saying, “I used to direct Muhammad wherever I willed. He would dictate to me ‘Most High, All-Wise’, and I would write down ‘All-Wise’ only. Then he would say, ‘Yes it is all the same’. On a certain occasion he said, ‘Write such and such’, but I wrote ‘Write’ only, and he said, ‘Write whatever you like.'” So when this scribe exposed Muhammad, he wrote in the Qur’an, “And who does greater evil than he who forges against God a lie, or says, ‘To me it has been revealed’, when naught has been revealed to him.” So on the day Muhammad conquered Mecca, he commanded his scribe to be killed. But the scribe fled to `Uthman Ibn `Affan, because `Uthman was his foster brother (his mother suckled `Uthman). `Uthman, therefore, kept him away from Muhammad. After the people calmed down, `Uthman brought the scribe to Muhammad and sought protection for him. Muhammad kept silent for a long time, after which he said yes. When `Uthman had left, Muhammad said “I only kept silent so that you (the people) should kill him.”” (Al-Sirah by al-‘Iraqi)
Reference needs verification

Ibn Ishaq (704-768 AD), the earliest seerah writer states:

“The apostle had instructed his commanders when they entered Mecca only to fight those who resisted them except a small number who were to be killed even if they were found beneath the curtains of the Kaaba. Among them was `Abdullah b. Sa`d, brother of the B. `Amir b. Lu’ayy. The reason he ordered him to be killed was that he had been a Muslim and used to write down revelation; then he apostatized and returned to Qurahysh [Mecca] and fled to `Uthman b. `Affan whose foster brother he was. The latter hid him until he brought him to the apostle after the situation in Mecca was tranquil, and asked that he might be granted immunity. They allege that the apostle remained silent for a long time till finally he said yes. When `Uthman had left he [Muhammad] said to his companions who were sitting around him, “I kept silent so that one of you might get up and strike off his head!” One of the Ansar [Muhammad’s helpers from Medina] said, “Then why didn’t you give me a sign, O apostle of God?” He [Muhammad] answered that a prophet does not kill by pointing.” (Sirat Rasul Allah by Ibn Ishaq, p. 550)

In Kitab Al-Tabaqat Al-Kabir by Ibn Sa’d (784-845 AD):

“A person of al-Ansar had taken a vow to kill Ibn Abi Sarh if he saw him. `Uthman whose foster brother he (Abi Sarh) was, came and interceded for him with the Prophet. The Ansari was waiting for the signal of the Prophet to kill him. `Uthman interceded and he [Muhammad] let him (Abi Sarh) go. Then the apostle of Allah said to the Ansari, “Why did you not fulfill your vow?” He said, “O apostle of Allah! I had my hand on the hilt of the sword waiting for your signal to kill him.” The Prophet said signaling would have been a breach of faith. “It does not behave a prophet to make a signal.” (Tabaqat Al-Kabir, p. 174)

And in several hadith in Abu Dawud, Muhammad talks about how he wished they killed him:

Narrated Sa’d: On the day when Mecca was conquered, the Apostle of Allah (peace be upon him) gave protection to the people except four men and two women and he named them. Ibn AbuSarh was one of them. He then narrated the tradition. He said: Ibn Abi Sarh hid himself with Uthman ibn Affan. When the Apostle of Allah (peace be upon him) called the people to take the oath of allegiance, he brought him and made him stand before the Apostle of Allah (peace be upon him). He said: Apostle of Allah, receive the oath of allegiance from him. He raised his head and looked at him thrice, denying him every time. After the third time he received his oath. He then turned to his companions and said: Is not there any intelligent man among you who would stand to this (man) when he saw me desisting from receiving the oath of allegiance, and kill him? They replied: We do not know, Apostle of Allah, what lies in your heart; did you not give us an hint with your eye? He said: It is not proper for a prophet to have a treacherous eye. (Abu Dawud 14:2677)

Narrated Abdullah ibn Abbas: Abdullah ibn AbuSarh used to write (the revelation) for the Apostle of Allah (peace be upon him). Satan made him slip, and he joined the infidels. The Apostle of Allah (peace be upon him) commanded to kill him on the day of Conquest (of Mecca). Uthman ibn Affan sought protection for him. The Apostle of Allah (peace be upon him) gave him protection. (Abu Dawud 38:4345)

Narrated Sa’d ibn AbuWaqqas: On the day of the conquest of Mecca, Abdullah ibn Sa’d ibn AbuSarh hid himself with Uthman ibn Affan. He brought him and made him stand before the Prophet (peace be upon him), and said: Accept the allegiance of Abdullah, Apostle of Allah! He raised his head and looked at him three times, refusing him each time, but accepted his allegiance after the third time. Then turning to his companions, he said: Was not there a wise man among you who would stand up to him when he saw that I had withheld my hand from accepting his allegiance, and kill him? They said: We did not know what you had in your heart, Apostle of Allah! Why did you not give us a signal with your eye? He said: It is not advisable for a Prophet to play deceptive tricks with the eyes.” (Abu Dawud 38:4346)

The Iranian rationalist Ali Dashti writes:

‘Abdollah b. Abi Sarh was a foster-brother of ‘Othman. He took refuge with ‘Othman, who kept him hidden for several days until the commotion subsided, and then brought him to the Prophet and requested pardon for him. After a long silence, the Prophet said, “Yes”, meaning that he reluctantly accepted ‘Othman’s intercession. Thereupon ‘Abdollah b. Abi Sarh professed Islam again and ‘Othman and he departed. The Prophet, when asked the reason for the long silence, replied, “His Islam was not voluntary but from fear, so I was reluctant to accept it. I was expecting one of you to stand up and behead him.” (This was because it had been proclaimed that his blood might be lawfully shed in any place where he might be found, “even if clinging to the covering of the K‘aba”). One of the Ansar asked the Prophet why he had not winked, and received the answer that “God’s Apostle cannot have false eyes”, meaning that he could not falsely pretend silence while giving a sign with the eyes to kill… (Dashti, 23 Years: A Study of the Prophetic Career of Mohammad, translated from the Persian by F.R.C. Bagley [Mazda Publishers, Costa Mesa, CA 1994], pp. 98-99)

Read more here: Answering Islam – Abdullah ibn Abi Sarh , Pdf: The Qur’an contains words by `Abdullah ibn Sa`d Ibn Abi Sarh