If We had sent down this Quran on a mountain, you would have seen it fallen down splitting asunder due to the fear of Allah. And these are similitudes We set forth for mankind that they may reflect*. [* See note under Chapter 25, Verse 32]
It is reported that when portions of the Quran were sent down on the messenger of Allah while on the camel back, the camel used to sit down due to the load. the messenger of Allah himself used to perspire[*]. (Seerat-un-Nabi by Syed Sulaiman Nadvi, 1928, Vol 3. P. 302-303). It was probably due to this reason that the whole Quran was not communicated all at once. It would probably not be possible for a human heart to bear the pressure which could crush a mountain. The first portion of the Quran consisted of only five verses (96 ; 1-5), then the second installment (Chapter:74) was communicated after a long interval, the whole Quran being communicated in small portions, in a span of twenty three years, probably to dissipate the tremendous pressure inherent in it.