Surah 4. An-Nisaa', Ayah 82

I.e., the fact that it is free of all inner contradictions - in spite of its having been revealed gradually, over a period of twenty-three years - should convince them that it has not been "composed by Muhammad" (an accusation frequently levelled against him not only by his contemporaries but also by non-believers of later times), but could only have originated from a supra-human source. See also 25:32 and 39:23 .

The unity of the Qur-an is admittedly greater than that of any other sacred book. And yet how can we account for it except through the unity of Allah's purpose and design? From a mere human point of view, we should have expected much discrepancy, because (1) the Messenger who promulgated it was not a learned man or philosopher, (2) it was promulgated at various times and in various circumstances, and (3) it is addressed to all grades of mankind. Yet, when properly understood, its various pieces fit together well even when arranged without any regard to chronological order. There was just the One Inspirer and the One Inspired.