VERILY, this Qur'an shows the way to all that is most uprightand gives the believers who do good deeds the glad tiding that theirs will be a great reward;
Al-Isra (The Journey by Night) Verse 9
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.
Sahih Bukhari: Volume 1, Book 3, Number 94