رياض الصالحينAbū Zakariyyā Muḥyī al-Dīn Yaḥyā ibn Sharaf al-Nawawī١٣٣ - باب آداب السلام
On the authority of Abū Hurayrah (may Allah be pleased with him): the Messenger of Allah (ﷺ) said, "The rider greets the one walking, the one walking greets the one sitting, and the smaller group greets the larger group." Agreed upon (al-Bukhārī and Muslim). In a narration by al-Bukhārī: "and the younger greets the older."
On the authority of Abū Umāmah Ṣudayy ibn ʿAjlān al-Bāhilī (may Allah be pleased with him), who said: the Messenger of Allah (ﷺ) said, "Indeed, the worthiest of people before Allah is the one who greets them first with the salām." Narrated by Abū Dāwūd with a good chain. Al-Tirmidhī also narrated it on the authority of Abū Umāmah (may Allah be pleased with him): it was asked, "O Messenger of Allah, when two men meet, which of them should begin with the salām?" He said, "The worthier of the two before Allah, the Exalted." Al-Tirmidhī said, "This is a good (ḥasan) hadith."