The following Quranic Ayah shows the Quran is Allah's words:
Dear believing brothers, in the Noble Quran, there are certain signs that are meant to call the reader's attention to certain cosmic facts, and the following Ayah is an example. Allah the Almighty says: 

(And proclaim to mankind the Hajj (pilgrimage). They will come to you on foot and on every lean camel, they will come from every deep and distant (wide) mountain highway (to perform Hajj).)

[Al-Hajj, 27]

It might be asked: "Why does Allah the Almighty say "from every deep way" and not "from every long way" in the above Ayah?" Scholars of the Quran interpretation hold that in the use of the word "deep" instead of "long" is a reference to the fact that the earth is round.  This is because lines on the earth's surface are not straight but rather bent; and in addition to the two dimensions of length and width a bent line needs a third dimension, i.e. depth.  Hence, by the above Ayah, Allah, Exalted and Sublime be He, desires to call our attention to the fact that the earth, on which we live, is round. However, the remarkable thing in this context is that the Noble Quran's wisdom conforms to the facts that were prevalent at the time when the Noble Quran was revealed as well as to those of later times.