Musnad Abu Ya'la al-Mawsili 1:62

Al-Qawariri narrated to us, Abd al-Rahman ibn Mahdi narrated to us, Ibrahim ibn Sa’d narrated to us, from al-Zuhri, from Ubayd ibn al-Sabbaq, that Zayd narrated to him, saying: Abu Bakr sent for me after the slaughter of the people of al-Yamamah, and behold, Umar ibn al-Khattab was with him. He said: "Umar came to me and said: 'The killing has intensified among the reciters of the Quran on the day of al-Yamamah, and I fear that it will intensify among the reciters in all regions, and much of the Quran will be lost. I see that you should order it to be collected.'" Abu Bakr said: "I said to Umar: 'How can I do something that the Messenger of Allah, peace and blessings of Allah be upon him, did not do?' Umar said: 'By Allah, it is good.' Umar ibn al-Khattab kept returning to me until Allah expanded my chest for that which He had expanded the chest of Umar, and I saw in it what he saw." Zayd said: 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, peace and blessings of Allah be upon him. So, track down the Quran and collect it." He said: "By Allah, if they had tasked me with moving a mountain from the mountains, it would not have been heavier upon me than that." I said: "How can you do something that the Messenger of Allah, peace and blessings of Allah be upon him, did not do?" Abu Bakr said: "By Allah, it is good." Abu Bakr and Umar kept returning to me regarding that until Allah expanded my chest for that which He had expanded the chest of Abu Bakr and Umar. So, I tracked down the Quran, collecting it from parchment, palm stalks, stones, thin pieces of leather, and from the chests of men. I found at the end of Surah al-Tawbah (Bara'ah) with Khuzaymah ibn Thabit: "There has certainly come to you a Messenger from among yourselves..." until the end of the verse.