Whenever the Prophet (ﷺ) spoke a sentence (said a thing), he used to repeat it thrice so that the people could understand it properly from him and whenever he asked permission to enter, (he knocked the door) thrice with greeting.
Reference: Sahih al-Bukhari 95 In-book : Book 3, Hadith 37 USC-MSA web (English) : Vol. 1, Book 3, Hadith 95 (deprecated)
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