Musnad Abu Ya'la al-Mawsili 3:37

Ubayd Allah ibn Umar narrated to us, Abd al-Wahid ibn Ziyad narrated to us, Abd al-Rahman ibn Ishaq narrated to us, from al-Nu'man ibn Sa'd, from Ali ibn Abi Talib, that he forbade a man from reciting the Quran while he is bowing (ruku'). And he said: "When you bow, then glorify Allah, and when you prostrate, then supplicate to Allah, for it is worthy that it be answered for you."