Musnad Abu Ya'la al-Mawsili 106:44

Al-Abbas ibn al-Walid al-Narsi narrated to us, Abu al-Ahwas narrated to us, Abu Ishaq narrated to us, from Abdullah ibn Yazid, who said: Al-Bara ibn Azib narrated to us—and he was not a liar—he said: We used to pray behind the Messenger of Allah, peace and blessings of Allah be upon him. When he said, "Allah hears those who praise Him," none of us would bend his back until the Prophet, peace and blessings of Allah be upon him, placed his forehead upon the ground. When he placed his forehead upon the ground, we would fall into prostration.