Musnad Ahmad 8:46

'Abd al-Rahman ibn Mahdi and Abu Sa'id narrated to us, they said: 'Abdullah ibn Ja'far narrated to us, from Isma'il ibn Muhammad. Abu Sa'id said: He said: Isma'il ibn Muhammad narrated to us, from 'Amir ibn Sa'd, from his father, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) — and Abu Sa'id said: I saw the Messenger of Allah (peace and blessings of Allah be upon him) — would perform the taslim to his right until the whiteness of his cheek was seen, and to his left until the whiteness of his cheek was seen."