Tafsir of At-Tariq 86:13

It is a word, meaning the Qur’an—of which these verses, speaking of the origin of man’s state and his return, are a part. This is more appropriate than making the pronoun refer to what preceded, meaning "what I have informed you of regarding My power to resurrect you," because the Qur’an encompasses that primarily, and the statement of the Exalted, "a decisive word," is more fitting for it.

The intended meaning is a word that distinguishes between truth and falsehood, having reached the ultimate limit in that, such that it is as if it were the act of distinction itself. It is also said that the juxtaposition of "decisive" (fasl) with "jest" (hazl) in what follows necessitates that it be interpreted as "definitive," meaning firmly established as truth. The first [interpretation] is superior.