Musnad Ahmad 1:56

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, may Allah be pleased with him, sent for me at the time of the slaughter of the people of al-Yamamah. 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 Quran and compile it."