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.''
App reference: Book 6, Hadith 13
Arabic/English book reference: Book 6, Hadith 857