Musnad Ahmad 1148:1

Yazid ibn Harun narrated to us, Ayyub ibn Utbah informed us, Abdullah ibn Badr narrated to us, he said: Abd al-Rahman ibn Ali ibn Shayban al-Suhaymi narrated to us, my father narrated to me: That he heard the Prophet, peace and blessings of Allah be upon him, saying: "Allah does not look at the prayer of a servant who does not straighten his back between his bowing (ruku') and his prostration (sujud)."