Musannaf Ibn Abi Shayba 3:183

Abu Bakr narrated to us, he said: Ibn Mushir and Ibn Fudayl narrated to us, from Abd al-Rahman ibn Ishaq, from al-Nu’man ibn Sa’d, from Ali, who said: The Prophet, peace and blessings of Allah be upon him, said: "I have been forbidden from reciting the Quran while bowing (ruku') and prostrating (sujud). So when you bow, glorify Allah, and when you prostrate, strive in supplication, for it is likely that it will be answered for you."