مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3676
Musnad of Abdullah bin Mas'ood
It was narrated from ‘Abdullah (رضي الله عنه) that The Prophet (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 cheek could be seen.
Musnad Ahmad 3699Its isnad is saheeh]