Abu Huraira reported: A man passed by the Messenger of Allah, peace and blessings be upon him, while he was sitting in a gathering. The man said, “Peace be upon you.” The Prophet said, “Ten good deeds.” Another man passed by and he said, “Peace be upon you and the mercy of Allah.” The Prophet said, “Twenty good deeds.” Another man passed by and he said, “Peace be upon you, the mercy of Allah, and his blessings.” The Prophet said, “Thirty good deeds.” A man stood up to leave the gathering and he did not offer salutations of peace. The Prophet said, “How quickly your companion forgets! If one of you comes to a gathering, let him greet them with peace. If there is room for him to sit, let him sit. If he stands to leave, let him offer salutations of peace. The former duty is no different than the latter.”
Source: al-Adab al-Mufrad 980
Grade: Sahih (authentic) according to Al-Albani
عَنْ أَبِي هُرَيْرَةَ أَنَّ رَجُلا مَرَّ عَلَى رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ وَهُوَ فِي مَجْلِسٍ فَقَالَ السَّلامُ عَلَيْكُمْ فَقَالَ عَشْرُ حَسَنَاتٍ فَمَرَّ رَجُلٌ آخَرُ فَقَالَ السَّلامُ عَلَيْكُمْ وَرَحْمَةُ اللَّهِ فَقَالَ عِشْرُونَ حَسَنَةً فَمَرَّ رَجُلٌ آخَرُ فَقَالَ السَّلامُ عَلَيْكُمْ وَرَحْمَةُ اللَّهِ وَبَرَكَاتُهُ فَقَالَ ثَلاثُونَ حَسَنَةً فَقَامَ رَجُلٌ مِنَ الْمَجْلِسِ وَلَمْ يُسَلِّمْ فَقَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ مَا أَوْشَكَ مَا نَسِيَ صَاحِبُكُمْ إِذَا جَاءَ أَحَدُكُمُ الْمَجْلِسَ فَلْيُسَلِّمْ فَإِنْ بَدَا لَهُ أَنْ يَجْلِسَ فَلْيَجْلِسْ وَإِذَا قَامَ فَلْيُسَلِّمْ مَا الأُولَى بِأَحَقَّ مِنَ الآخِرَةِ
980 الأدب المفرد للبخاري باب فضل السلام
757 المحدث الألباني خلاصة حكم المحدث صحيح في صحيح الأدب المفرد