Musnad Abu Ya'la al-Mawsili 3:156

Masruq ibn al-Marzuban narrated to us, Yahya ibn Zakariyya narrated to us, from Abd al-Rahman ibn Ishaq: Al-Nu'man ibn Sa'd narrated to me, he said: We were with Ali, and a man asked him, "Should I recite [the Quran] while bowing or while prostrating?" Ali said: The Messenger of Allah (may Allah bless him and grant him peace) said: "I have been forbidden to recite [the Quran] while bowing or while prostrating. So when you bow, glorify Allah, and when you prostrate, strive in supplication, for it is likely that it will be answered for you."