Musannaf Ibn Abi Shayba 3:4772

Hushaym narrated to us, he said: Al-Awwam informed us, from 'Uzrah ibn al-Harith, that he narrated to him from al-Bara' ibn 'Azib, who said: When we used to pray behind the Messenger of Allah (peace and blessings of Allah be upon him), and we raised our heads from bowing, we would stand in rows until he prostrated. Then, when he prostrated, we would follow him.