サヒーフ・アル＝ブハーリー #95
Sahih (Authentic)
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
伝承の連鎖（イスナード）
Narrated Anas