Musnad Ahmad 25:555

Abu Sa’id, the freed slave of Banu Hashim, and ‘Abd al-Samad—the meaning being the same—said: Thabit narrated to us, ‘Asim narrated to us, from al-Sha’bi, from Ibn ‘Abbas, who said: "I stood to pray with the Prophet, peace and blessings of Allah be upon him, and I stood to his left. He reached with his hand from behind him until he took hold of my upper arm or my hand, until he positioned me to his right."