Musnad Ahmad 658:238

Waki’ narrated to us, from Sufyan, from Abu Ishaq, from ‘Abdullah ibn Yazid, from Al-Bara’ who said: "When the Messenger of Allah (peace and blessings of Allah be upon him) would raise his head from bowing, not a single man among us would bend his back until he (the Prophet) had prostrated, then we would prostrate."