Musannaf Ibn Abi Shayba 3:667

Hussain bin Ali narrated to us, from Za'idah, from 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 in the prayer to his right and to his left, until the whiteness of his cheek could be seen. And he would say, "As-salamu alaykum wa rahmatullah" (Peace be upon you and the mercy of Allah) from both sides.