Musnad Abu Ya'la al-Mawsili 152:34

On the authority of Abu Ishaq, from Abu al-Ahwas, 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: "As-salamu alaykum wa rahmatullah" (Peace be upon you and the mercy of Allah), "As-salamu alaykum wa rahmatullah" (Peace be upon you and the mercy of Allah), until the whiteness of his cheeks could be seen.