Musnad Abu Ya'la al-Mawsili 106:24

Zakariya ibn Yahya narrated to us, Hushaym narrated to us, from al-Awwam, 'Uzrah ibn al-Harith al-Shaybani informed me, from al-Bara' who said: When we used to pray with the Messenger of Allah (may Allah bless him and grant him peace), and we raised our heads from bowing, we would stand in rows until the Prophet (may Allah bless him and grant him peace) prostrated. Then, when he prostrated, we would follow him.