Musnad Ahmad 938:69

Abu Kamil narrated to us, Ibrahim ibn Sa’d narrated to us, Ibn Shihab narrated to us, from ‘Ubayd ibn al-Sabbaq, from Zayd ibn Thabit, who said: "Abu Bakr sent for me at the time of the slaughter of al-Yamamah, and behold, ‘Umar was sitting with him. Abu Bakr said: 'O Zayd ibn Thabit, 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, track down the Qur’an 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 Qur’an. I said: 'Will you both do something that the Messenger of Allah, may Allah bless him and grant him peace, did not do?' He said: 'It is, by Allah, good.' Abu Bakr continued to urge me until Allah expanded my chest for that which He had expanded the chests of Abu Bakr and ‘Umar, may Allah be pleased with them both."