Musnad Ahmad 3680

It was narrated that ‘Abdullah (رضي الله عنه) said: I have not forgotten, among the things I forgot, that The Messenger of Allah (ﷺ) used to say the salam to his right and to his left, "As-salamu 'alaikum wa rahmatullah, as-salamu ‘alaikum wa rahmatullah," until the whiteness of his cheeks could be seen - or until we could see the whiteness of his cheeks.