Hadith 17:402
Abu Usama narrated to us, from Abd al-Mu'min, who said: I greeted a man who was walking with Muslim ibn Yasar, and I said, "Peace be upon you" (al-salamu 'alayk). Muslim said to me, "Hush." I said, "I know him." He said, "Even so. If you greet, then say, 'Peace be upon you all' (al-salamu 'alaykum), for there are guardians (angels) with him."
حدثنا أبو أسامة عن عبد المؤمن قال سلمت على رجل يمشي مع مسلم بن يسار فقلت السلام عليك فقال لي مسلم مه فقلت إني عرفته فقال وإن إذا سلمت فقل السلام عليكم فإن معه حفظة