Tafsir of Al-Mursalat 77:7

{Indeed, what you are promised is to occur}

"Indeed, what you are promised is to occur" is the response to the oath. "Ma" (what) is a relative pronoun, and even though it is written conjoined [with the preceding word], the referential pronoun (the 'a'id) is omitted. That is to say: that which you are promised regarding the coming of the Resurrection is something that will inevitably happen. It is also permissible that the intended meaning of the relative pronoun is everything included in the preceding chapter, though this is highly contrary to the apparent meaning.