Musnad Ahmad 658:55

Isma'il narrated to us, he said: Shu'bah narrated to us, from Abu Ishaq, he said: I heard 'Abdullah ibn Yazid delivering a sermon, and he said: Al-Bara' narrated to us—and he was not a liar—"That they used to, when they prayed with the Messenger of Allah (may Allah bless him and grant him peace), and he raised his head from the bowing (ruku'), they would remain standing until they saw him prostrating, then they would prostrate."