And when Moses arrived at Our appointed time and his Lord spoke to him, he said, "My Lord, show me [Yourself] that I may look at You." [Allāh] said, "You will not see Me,1 but look at the mountain; if it should remain in place, then you will see Me." But when his Lord appeared to the mountain, He rendered it level,2 and Moses fell unconscious. And when he awoke, he said, "Exalted are You! I have repented to You, and I am the first [among my people] of the believers."3
Translation: Saheeh International (English)
1 That no one can see Allāh during the life of this world.
2 It crumbled to dust.
3 This is a severe warning from Allāh against rebellion.
When Moses came to meet his Lord for the appointment he had been assigned, which was a full forty nights, and his Lord spoke to him giving him instructions and prohibitions and so on, Moses desired to see his Lord, so he asked to see Him. Allah the Exalted replied: 'You will not see Me in the life of this world, because you will not be able to endure that, but look instead at the mountain when I reveal Myself to it, if it remains in its place and is not affected then You will be able to see Me; but if it becomes flattened, then you will not be able to see Me in the life of this world.' Then when Allah revealed Himself to the mountain it crumbled to dust, and Moses fell down unconscious. When he recovered from his unconsciousness, he said: 'I declare Your Perfection O Lord, and that You are far above any deficiency being ascribed to You, and I turn to You asking for forgiveness for having asked to see You in this world, and I am the first of my people to believe.'