Musnad Ahmad 1:75

'Uthman ibn 'Umar narrated to us, he said: Yunus informed us, from al-Zuhri, he said: Ibn al-Sabbaq informed me, he said: Zayd ibn Thabit informed me: That Abu Bakr sent for him after the slaughter of the people of al-Yamamah, and 'Umar was with him. Abu Bakr said: "'Umar came to me and said: 'The slaughter has intensified among the people of al-Yamamah from the reciters of the Quran among the Muslims, and I fear that the slaughter will intensify among the reciters in other places, and much of the Quran will be lost, not being 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?' He 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. You used to write the revelation for the Messenger of Allah, may Allah bless him and grant him peace, so 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?'"