Musnad Ahmad 1148:3

'Abd al-Samad narrated to us, my father narrated to me, Abu 'Abd Allah al-Shaqari narrated to us, 'Umar ibn Jabir narrated to me, from 'Abd Allah ibn Badr, from 'Abd al-Rahman ibn 'Ali, from his father, he said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "Indeed, Allah does not look at a man who does not straighten his back in his bowing (ruku') and in his prostration (sujud)."