Musannaf Ibn Abi Shayba 3:580

Mulazim ibn ‘Amr ibn ‘Abd Allah ibn Badr narrated to us, he said: ‘Abd al-Rahman ibn ‘Ali ibn Shayban narrated to me, from his father, ‘Ali ibn Shayban—and he was among the delegation—he said: We set out until we arrived at the Prophet of Allah, may Allah bless him and grant him peace, and we pledged allegiance to him and prayed with him. He glanced with the corner of his eye at a man who was not straightening his back in the bowing (ruku‘) and the prostration (sujud). When the Prophet, may Allah bless him and grant him peace, finished the prayer, he said: "O assembly of Muslims, there is no prayer for a person who does not straighten his back in the bowing and the prostration."