Musnad Ahmad 25:1500

'Abd al-Rahman ibn Mahdi narrated to us, from Sufyan, from al-A'mash, who said: I asked Ibrahim about a man who prays with the Imam. He said: "He stands to his left." I said: "Sami' al-Zayyat narrated to me, saying: I heard Ibn Abbas narrating: 'That the Prophet, peace and blessings of Allah be upon him, made him stand to his right, and he took hold of him.'"