Silsilah as-Sahihah 2644 — Etiquette of Salam

It is narrated from Sayyiduna Abu Sa'id Khudri, may Allah be pleased with him, who says that the Messenger of Allah, peace and blessings be upon him, said: "When some people pass by a group and one of those passing greets (with salam), and one of those sitting replies, then it will suffice for both sides."