Musannaf Ibn Abi Shayba 3:157

Abu Bakr narrated to us, he said: Isma'il bin 'Ayyash narrated to us, from 'Abd al-'Aziz bin 'Ubayd Allah, from Abu Ja'far, from 'Ali, who said: When you bow, place your palms upon your knees and flatten your back. Do not raise your head, and do not lower it. Do not stretch out, and do not contract.