(30) باب مَنْ أَعَادَ الْحَدِيثَ ثَلاَثًا لِيُفْهَمَ عَنْهُ
Narrated Anas:
Whenever the Prophet (ﷺ) asked permission to enter, he knocked the door thrice with greeting and whenever he spoke a sentence (said a thing) he used to repeat it thrice. (See Hadith No. 261, Vol. 8).
Reference: Sahih al-Bukhari 94
In-book reference: Book 3, Hadith 36
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 1, Book 3, Hadith 94