Narrated Anas: Whenever the Prophet 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).
Hadith ReferenceSahih Bukhari / The Book of Knowledge / Hadith: 94
Hadith Gradingمحدثین:أحاديث صحيح البخاريّ كلّها صحيحة