Riyad as-Salihin: The Book of Greetings – Hadith No. 178 — Abu Hurairah (May Allah be pleased with him) reported: The Messenger of Allah… (Hadith No. 178)

Abu Hurairah (May Allah be pleased with him) reported: The 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).''

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The narration in Al-Bukhari adds: The Messenger of Allah (ﷺ) said, "The young should greet the elderly.''

Explanation:

Reference: Riyad as-Salihin, The Book of Greetings, Hadith No. 178