Musnad Ahmad 5:40

Abdullah narrated to us, Hajjaj ibn Yusuf al-Sha'ir narrated to me, Yahya ibn Hammad narrated to us, Abu 'Awanah narrated to us, from 'Ata' ibn al-Sa'ib, from Musa ibn Salim Abu Jahdam, that Abu Ja'far narrated to him, from his father, that 'Ali narrated to them: "The Messenger of Allah, may Allah bless him and grant him peace, forbade me from three things—he said: 'And I do not know if it was specifically for him, or generally for the people'—he forbade me from [wearing] silk-blend garments (al-qassi), [using] the red saddle-cloth (al-maytharah), and that I should recite [the Qur'an] while I am bowing."