Musnad Abu Ya'la al-Mawsili 117:86

Hadbata narrated to us, Hammad narrated to us, from Thabit, from Anas, who said: I have never prayed behind anyone more concise in prayer than the Messenger of Allah (may Allah bless him and grant him peace), while it was complete. His prayer was measured, and the prayer of Abu Bakr was measured. When it was the time of Umar, he would lengthen the dawn prayer. When the Messenger of Allah (may Allah bless him and grant him peace) would say, "Allah hears those who praise Him," he would stand until we would say, "He has forgotten," then he would prostrate. And he would sit between the two prostrations until we would say, "He has forgotten."