Musnad Ahmad 32:1633

'Affan narrated to us, Hammad ibn Salamah narrated to us, Thabit informed us, from Anas, that he said: "I have never prayed behind a man who prayed a more concise prayer than the Messenger of Allah, may Allah bless him and grant him peace, [while it was] consistent. And the prayer of Abu Bakr was consistent. But when it was the time of 'Umar, he would lengthen the dawn prayer." He said: "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.' And he would sit between the two prostrations until we would say, 'He has forgotten.'"