Sahih Adab al-Mufrad

761/986- (Sahih) From Abu Huraira, that a man passed by the Messenger of Allah, peace be upon him, while he was in a gathering, and said: "Peace be upon you." He said: "Ten good deeds." Then another man passed by and said: "Peace be upon you and the mercy of Allah." He said: "Twenty good deeds." Then another man passed by and said: "Peace be upon you and the mercy of Allah and His blessings." He said: "Thirty good deeds." Then a man from the gathering stood up and did not give salam! The Messenger of Allah, peace be upon him, said: "How quickly your companion forgot! When one of you comes to a gathering, let him give salam, and if it appears to him to sit, let him sit, and if he stands up (and in another narration: if he sits and then it appears to him to stand before the gathering disperses/__QUR_N__), let him give salam; the first is not more deserving than the last."