Musannaf Ibn Abi Shayba 3:4778

Waki’ narrated to us, from Sufyan, from Abu Ishaq, from ‘Abdullah ibn Yazid, from Al-Bara’ ibn ‘Azib, who said: When the Messenger of Allah (peace and blessings of Allah be upon him) would raise his head from bowing (ruku’), none of us would bend our backs until he prostrated. Then, when he prostrated, we would follow him.