(30) باب مَنْ أَعَادَ الْحَدِيثَ ثَلاَثًا لِيُفْهَمَ عَنْهُ
Narrated Anas:
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 reference: Book 3, Hadith 37
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 1, Book 3, Hadith 95