Musnad Ahmad 26:152

Waki' narrated to us, Sufyan narrated to us, from Abu Ishaq, from Abu al-Ahwas, from Abdullah: That the Prophet, peace and blessings of Allah be upon him, used to say the taslim to his right and to his left: "As-salamu 'alaykum wa rahmatullah, As-salamu 'alaykum wa rahmatullah," until the whiteness of his cheek could be seen.