Musnad Ahmad 658:50

'Affan narrated to us, Shu'bah narrated to us, he said: Abu Ishaq informed me, he said: I heard 'Abdullah bin Yazid delivering a sermon: 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 that he had prostrated, then they would prostrate."