Musnad Ahmad 5:767

Abdullah narrated to us, Suwayd ibn Sa'id narrated to me, in the year two hundred and twenty-six, Ali ibn Mushir informed us, from Abd al-Rahman ibn Ishaq, from al-Nu'man ibn Sa'd, from Ali, may Allah be pleased with him, who said: A man asked him, "Should I recite in the bowing (ruku') and the prostration (sujud)?" He said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "I have been forbidden from reciting in the bowing and the prostration. So when you bow, then glorify Allah, and when you prostrate, then strive in supplication, for it is likely that it will be answered for you."