Musnad Ahmad 32:1425

Yunus narrated to us, Hammad—meaning Ibn Zayd—narrated to us, from Thabit, from Anas ibn Malik, that he said: "I do not fall short in praying with you as the Messenger of Allah (peace and blessings of Allah be upon him) used to pray with us." He said: So Anas used to do something that I do not see you doing. When he raised his head from bowing, he would stand upright until a person would say: "He has forgotten." And when he raised his head from prostration, he would sit until a person would say: "He has forgotten."