Verily, those who have attained to faith and do good works, and are constant in prayer, and dispense charity - they shall have their reward with their Sustainer, and no fear need they have, and neither shall they grieve.
Al-Baqara (The Cow) Verse 277
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