Musannaf Ibn Abi Shayba 3:210

Abu Bakr narrated to us, he said: Ibn ‘Uyaynah narrated to us, from ‘Abdullah bin ‘Uthman, from a man from Thaqif, who said: I asked Abu Hurayrah, and he said: "Avoid the *hanwah* (bending/curving) in *ruku’* (bowing), and the *hadbah* (hunching)."