Musannaf Ibn Abi Shayba 3:579

Abu Bakr narrated to us, he said: Abu Mu'awiyah and Waki' narrated to us, from al-A'mash, from 'Umarah, from Abu Ma'mar, from Abu Mas'ud, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "A prayer in which a man does not straighten his back during the bowing (ruku') and the prostration (sujud) is not sufficient."