Musnad Abu Ya'la al-Mawsili 1:69

Ubayd Allah ibn Umar al-Qawariri narrated to us, Uthman ibn Umar narrated to us, Yunus ibn Yazid narrated to us, from al-Zuhri, Ibn al-Sabbaq informed me, Zayd ibn Thabit informed me: That Abu Bakr sent for him after the slaughter of the people of al-Yamamah. He said: So I came to him, and Umar was with him. Abu Bakr said: "Umar came to me and said: 'The slaughter has intensified among the people of al-Yamamah—among the reciters of the Quran or the people' (Abu Ya'la was in doubt). 'I fear that the slaughter will intensify in all regions, and much of the Quran will be lost if it is not preserved. I see that you should order the collection of the Quran.' I said to Umar: 'How can I do something that the Messenger of Allah, may Allah bless him and grant him peace, did not do?' Umar said: 'By Allah, it is good.' He continued to urge me regarding that until Allah expanded my chest for it, and I saw in it what Umar saw." Zayd said, and Umar was sitting with him, not speaking: Abu Bakr said: "You are a young, intelligent man, and we do not suspect you, and you used to write the revelation for the Messenger of Allah, may Allah bless him and grant him peace. So follow the Quran and collect it." Zayd said: "By Allah, if they had tasked me with moving a mountain from the mountains, it would not have been heavier upon me than what he ordered me to do regarding the collection of the Quran. I said: 'How can you do something that the Messenger of Allah, may Allah bless him and grant him peace, did not do?' He said: 'By Allah, it is good.' He continued to urge me until Allah expanded my chest for that which He expanded the chests of Abu Bakr and Umar for." So I collected the Quran, tracing it from scraps, shoulder blades, and the chests of men, until I found the end of Surah al-Tawbah with Khuzaymah al-Ansari; I did not find it with anyone else: "There has certainly come to you a Messenger from among yourselves..." to the end of the verse. The codices in which we collected the Quran remained with Abu Bakr during his life until Allah took his soul, then with Umar until Allah took his soul, then with Hafsah.