HAD WE bestowed this Qur’an from on high upon a mountain, thou wouldst indeed see it humbling itself, breaking asunder for awe of God. And [all] such parables We propound unto men, so that they might [learn to] think.
Al-Hashr (The Gathering) Verse 21
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