مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3856
Musnad of Abdullah bin Mas'ood
It was narrated that `Abdullah (رضي الله عنه) said: The Messenger of Allah (ﷺ) used to say the salam to his right and to his left until the whiteness of his cheek could be seen: As-salamu ‘alaikum wa rahmatullah,"
Musnad Ahmad 3879A sahạeh hadeeth]