Musnad Ahmad 32:817

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Thabit, who said: I heard Anas ibn Malik say: "He used to describe the prayer of the Messenger of Allah (peace and blessings of Allah be upon him) to us, then he would stand and pray. When he raised his head from bowing, we would say: 'He has forgotten,' due to the length of time he would remain standing."