Al-Tirmidhī, in the narration he transmits from Zayd, states:
“​I discovered the closing passage of Sūrat al-Barāʾah in the possession of Khuzaymah ibn Thābit:
‘There has certainly come to you a Messenger from among yourselves; grievous to him is your distress; he is deeply concerned for you, and to the believers he is gentle and merciful. But if they turn away, say, ‘Allah is sufficient for me; there is no deity but Him. In Him I place my trust, and He is the Lord of the Mighty Throne.’’ ”
He then remarked: “This ḥadīth is ḥasan ṣaḥīḥ.”

Al-Bukhārī records from Zayd ibn Thābit who said:
“When we copied the parchments into codices, I missed a verse from Sūrat al-Aḥzāb which I had heard the Messenger of Allah (peace and blessings be upon him) recite. I found it with no one but Khuzaymah al-Anṣārī—whose single testimony the Messenger of Allah (peace and blessings be upon him) accepted as equal to that of two men—
‘Among the believers are men who were true to the covenant they made with Allah.’ ”

Al-Tirmidhī also narrates from him:
“I lost a verse from Sūrat al-Aḥzāb which I had heard the Messenger of Allah (peace and blessings be upon him) recite:
‘Among the believers are men who were true to the covenant they made with Allah; some of them have fulfilled their vow and some are still waiting …’
I searched for it until I found it with Khuzaymah ibn Thābit—or Abū Khuzaymah—so I inserted it in its sūrah.”

I say: Thus the first of the two concluding verses of Barāʾah was absent in the first compilation, as related by al-Bukhārī and al-Tirmidhī, and in the second compilation a verse from Sūrat al-Aḥzāb was missing.

Al-Ṭabarī reports that the verse of Barāʾah fell out in the last compilation, but the former view is more sound—and Allah knows best.

If it is asked, “Why did ʿUthmān gather the people upon his codex when Abū Bakr had already compiled the Qur’an before him?” the answer is that ʿUthmān—may Allah be pleased with him—did not intend to produce a new compilation. Do you not see how he wrote to Ḥafṣah, “Send us the parchments so that we may copy them into codices and then return them to you,” as will be mentioned? Rather, ʿUthmān acted because people had begun to differ in their readings after the Companions had dispersed throughout the provinces. The matter became serious: their differences grew, each group clinging to its own rendering, until what Ḥudhayfah—may Allah be pleased with him—described occurred between the people of Syria and Iraq. They had assembled during the campaign of Armenia; every contingent recited according to what it had received, whereupon they disagreed, argued, and some even branded others as unbelievers, disavowing them and invoking curses. Alarmed by what he saw, Ḥudhayfah hastened to Madinah—as related by al-Bukhārī and al-Tirmidhī—entered upon ʿUthmān before going to his own house, and said:
“Rescue this Ummah before it is ruined!”
He asked, “Over what?”
He replied, “Over the Book of Allah; I have witnessed…"