A man passed by the Messenger of Allah ﷺ while he was sitting in a gathering, and said, "Peace be upon you."The Prophet ﷺ 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 stood up from the gathering and left without giving the greeting of peace, so the Prophet ﷺ said, "How quickly your companion has forgotten! When one of you comes to a gathering, let him give the greeting of peace. Then if he wishes to sit, let him sit. And when he gets up to leave, let him give the greeting of peace, for the first is no more deserving than the last."
أن رجلا مر على رسول الله ﷺ وهو في مجلس، فقال: سلام عليكم. فقال: عشر حسنات ثم مر رجل آخر فقال: سلام عليكم ورحمة الله. فقال: عشرون حسنة فمر رجل آخر فقال: سلام عليكم ورحمة الله وبركاته. فقال: ثلاثون حسنة. فقام رجل من المجلس ولم يسلم، فقال النبي ﷺ: ما أوشك ما نسي صاحبكم! إذا جاء أحدكم إلى المجلس فليسلم، فإن بدا له أن يجلس فليجلس، فإن قام فليسلم، فليست الأولى بأحق من الآخرة.
Bukhari narrated it in Adab al-Mufrad (986) from Abdul-Aziz bin Abdullah, who said, "Muhammad bin Jafar bin Abu Kathir told me, from Yaqub bin Zayd Taymi, from Saeed Maqburi, from Abu Hurairah," and he related it.Ibn Hibban graded it sahih (493), and he narrated it from Umar bin Muhammad Hamdani, who said, "Muhammad bin Ismail Bukhari told us," and he related it, and its chain of transmission is sahih.