مُسْنَدُ أَحْمَدَ
Musnad Ahmad 4252
Musnad of Abdullah bin Mas'ood
It was narrated that 'Abdullah said: The Messenger of Allah (ﷺ) used to say salam to his right (turning his face so far that) the whiteness of his cheek could be seen, saying, "As-salamu 'alaikum wa rahmatullah"; and to his left (turning his face so far that the whiteness of his cheek could be seen, saying, "As-salamu ‘alaikum wa rahmatullah."
Musnad Ahmad 4280Its isnad is saheeh]