The Messenger of Allah ﷺ used to give the salutation to his right and to his left until the whiteness of his cheek could be seen, saying, "Peace be upon you and the mercy of Allah."
حدثنا حميد بن عبد الرحمن حدثنا الحسن عن أبي إسحاق حدثنا أبو الأحوص
كان رسول الله ﷺ يسلم عن يمينه وعن يساره حتى يرى بياض خده السلام عليكم ورحمة الله
Its isnad is sahih. Humayd bin Abdul-Rahman Ruwasi is trustworthy, one of the teachers of Ahmad. He was deemed trustworthy by Ahmad, Ibn Main, and others, and Abu Bakr bin Abu Shayba said, "I have rarely seen anyone like him." Bukhari included his biography in al-Kabir, 1/2/344. Hasan is Ibn Salih bin Salih bin Hayy. This hadith is a repetition of 3849.