Musnad Abu Ya'la al-Mawsili 110:244

Zuhayr narrated to us, Ahwas ibn Jawwab al-Dabbi narrated to us, Ammar ibn Ruzayq narrated to us, from al-A’mash, from Sumay’, the freed slave of Ibn Abbas, from Ibn Abbas, who said: I stood with the Prophet, may Allah bless him and grant him peace, in prayer on his left side. So he took my hand and placed me on his right side.