Musannaf Ibn Abi Shayba 27:326

Abu Mu'awiyah narrated to us, he said: Al-A'mash narrated to us, from Abu Ishaq, from Zayd ibn Yuthay', he said: Abu Bakr was with the Messenger of Allah, may Allah bless him and grant him peace, on the day of Badr in the booth (al-'arish).