Musannaf Ibn Abi Shayba 35:247

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