Musnad Abu Ya'la al-Mawsili 1:63

Abu Khaythama narrated to us, Ya'qub ibn Ibrahim narrated to us, my father narrated to us, from Ibn Shihab, from 'Ubayd ibn al-Sabbaq, that Zayd ibn Thabit narrated to him that 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, track down the Qur'an and compile it."