Translation:

21. Had We sent down this Qur'an on a mountain verily thou would have seen it humble itself and cleave asunder for fear of Allah such are the similitudes which We propound to men that they may reflect.

Notes (Tafseer)

5398. There are two ideas associated in men's minds with a mountain: one is its height, and the other that it is rocky, stony, hard, Now comes the metaphor. The Revelation of Allah is so sublime that even the highest mountains humble themselves before it. The Revelation is so powerful and convincing that even the hard rock splits asunder under it. Will man then be so arrogant as to consider himself superior to it, or so hard-hearted as not to be affected by its powerful Message? The answer is "No" for unspoilt man; "Yes" for man when degraded by sin to be the vilest of creatures.

5399. Cf. vii. 143, and n. 1103, where, in the story of Moses, the Mount became as dust "when the Lord manifested His Glory". Also cf. xxxiii. 72, and n. 3778, where the mountains are mentioned as an emblem of stability, but as refusing to accept the Trust (Amanat) because they felt themselves to be too humble to be equal to such a tremendous Trust.