Abu Hurairah (May Allah be pleased with him) reported:
Messenger of Allah (ﷺ) said, 'A rider should greet a pedestrian; a pedestrian should greet one who is sitting; and a small group should greet a large group (of people).''[Al-Bukhari and Muslim].The narration in Al-Bukhari adds: Messenger of Allah (ﷺ) said, "The young should greet the elderly.'' (AR)
App reference (AR): Book 6, Hadith 13 (AR)
Arabic/English book reference (AR): Book 6, Hadith 857 (AR)