Musnad Ahmad 658:185

'Abd al-Rahman narrated to us, from Sufyan, from Abu Ishaq, from 'Abdullah ibn Yazid, who said: Al-Bara' narrated to us—and he was not a liar—he said: "When we used to pray behind the Messenger of Allah, may Allah bless him and grant him peace, and he would raise his head from bowing, not one of us would bend his back until the Prophet, may Allah bless him and grant him peace, would prostrate, and then we would prostrate."