Musnad Ahmad 30:3668

Yahya ibn Adam narrated to us, 'Amir ibn Yasaf narrated to us, Yahya ibn Abi Kathir narrated to us, from 'Abdullah ibn Badr al-Hanafi, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Allah does not look at the prayer of a man who does not straighten his back between his bowing and his prostration."