Musnad Ahmad 32:1161

Yazid narrated to us, Hammad ibn Salamah informed us, from Thabit, from Anas, who said: "When the Messenger of Allah (peace and blessings of Allah be upon him) would raise his head from bowing, he would stand until we would say: 'He has forgotten.' And when he would raise his head from the prostration, he would sit between the two prostrations until we would say: 'He has forgotten.'"