Musnad Ahmad 457:11

Husayn ibn Muhammad narrated to us, Shu'bah narrated to us, from Sulayman, who said: I heard 'Umarah ibn 'Umayr al-Taymi narrating from Abu Ma'mar al-Azdi, from Abu Mas'ud, from the Prophet, peace and blessings of Allah be upon him, who said: "The prayer of a man, or anyone, does not suffice if he does not straighten his back in the bowing (ruku') and the prostration (sujud)."