Musnad Ahmad 26:329

Humaid ibn Abd al-Rahman narrated to us, al-Hasan narrated to us, from Abu Ishaq, Abu al-Ahwas narrated to us, from Abdullah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) used to perform the taslim to his right and to his left, until the whiteness of his cheek could be seen: "As-salamu alaykum wa rahmatullah" (Peace be upon you and the mercy of Allah).