When Prophet Moses said, 'My Lord, show Thyself to me that I may look at Thee.' ALLAH replied, 'Thou shalt not see ME.

"And when Moses came at Our appointed time and his Lord spoke to him, he said, 'My Lord, show Thyself to me that I may look at Thee.' ALLAH replied, 'Thou shalt not see ME. But look at the mountain; and if it remains in its place, then thou shalt see ME.' And when his Lord manifested Himself on the mountain, HE broke it into pieces and Moses fell down unconscious. And when he recovered, he said: Glory unto Thee! I turn unto Thee repentant, and I am the first to believe". (7:143)

http://www.comp.leeds.ac.uk/nora/html/7-143.html