Tafsir of Al-Mutaffifeen 83:18

{Nay!} A rebuke against their denial.

{Indeed, the record of the righteous...} That is, what has been written of their deeds.

{...is in 'Illiyyun} A proper name for the register of good, in which is recorded everything done by the angels and the righteous among the two species (jinn and mankind). It is derived from ‘uluww (loftiness), in the form fa‘īl (like sijjīn is derived from sijn). It is named so either because it is the cause for rising to the highest degrees in Paradise, or because it is raised in the seventh heaven, where the Cherubim dwell, as an honor and exaltation for it.

It is narrated: "Indeed, the angels ascend with the servant’s deed, and they deem it little. When they reach with it to where Allah wills of His dominion, He reveals to them: 'You are the guardians over My servant, but I am the Watcher over what is in his heart. He has made his deed sincere for Me, so place it in ‘Illiyyīn, for I have forgiven him.' And indeed, they ascend with the servant’s deed, and they praise it. When they reach with it to where Allah wills, it is revealed to them: 'You are the guardians over My servant, but I am the Watcher over what is in his heart. He did not make his deed sincere for Me, so place it in Sijjīn.'"

{Indeed, the righteous will be in pleasure. On adorned couches, observing. You will recognize in their faces the radiance of pleasure. They will be given to drink of a sealed nectar. Its seal is musk, and for that let the competitors compete. And its mixture is from Tasnim, a spring from which those brought near will drink.}