Behold, those who are bent on denying the truth and on turning others away from the path of God have indeed gone far astray.
An-Nisa (The Women) Verse 167
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