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).
Sahih al-Bukhari Book 3, Hadith 36
← Previous Hadith One day Allah's Messenger (ﷺ) came out (before the people) and `Abdullah bin Hudhafa stood up and asked (him) "Who is my father?" The Prophet (ﷺ) replied, "Your father is Hudhafa." The Prophet (ﷺ) told them repeatedly (in anger) to ask him anything they liked. `Umar knelt down before the Prophet (ﷺ) and said thrice, "We accept Allah as (our) Lord and Islam as (our) religion and Muhammad as (our) Prophet." After that the Prophet (ﷺ) became silent.

Sahih al-Bukhari Book 3, Hadith 35 Next Hadith → 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.

Sahih al-Bukhari Book 3, Hadith 37