サヒーフ・アル＝ブハーリー #94
Sahih (Authentic)
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)
伝承の連鎖（イスナード）
Narrated Anas