Musnad Ahmad 26:730

'Umar ibn 'Ubayd narrated to us, 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 to his right until the whiteness of his cheek appeared, saying: "As-salamu 'alaykum wa rahmatullah." And to his left until the whiteness of his cheek appeared, saying: "As-salamu 'alaykum wa rahmatullah."