Riyad as-Salihin #173
The Book of Greetings
The Arabic includes the chain of narration (sanad) and the collector's commentary; the English translation shows the Prophet's words only.
وعن أنس رضي الله عنه أن النبي صلى الله عليه وسلم، كان إذا تكلم بكلمة أعادها ثلاثًا حتى تفهم عنه، وإذا أتى على قوم فسلم عليهم سلم عليهم ثلاثًا. ((رواه البخاري))
وهذا محمول على ما إذا كان الجمع كثيرًا.
Anas (May Allah be pleased with him) reported the Prophet (ﷺ) used to repeat his words thrice so that the meaning thereof would be fully understood, and whenever he came upon a gathering of people, he would greet them. He would repeat Salam thrice..
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About Riyad as-Salihin
This hadith is #173 from Riyad as-Salihin, in the Book of The Book of Greetings.