Tafsir of Al-Hijr 15:64

Two interpretations are permitted regarding the ba (ب) in His, the Exalted’s, saying: "And We have brought you the truth" (wa ataynaka bil-haqq). The first is that it refers to the matter that is certain and verified, in which there is no room for doubt or skepticism, which is their punishment. It was expressed this way to explicitly state the negation of any doubt regarding it. It is also permitted that "the truth" (al-haqq) refers to the news regarding the arrival of the aforementioned punishment.

His saying, the Exalted: "And indeed, We are truthful"

This is a confirmation of it; that is, "We have brought to you, in what we have said, the truthful report," meaning that which corresponds to reality, "and indeed We are truthful" in that report, or in every report, such that it serves as evidence for their veracity regarding it. According to the first interpretation, it is a confirmation after a confirmation. Some people have permitted the possibility that the ba is for concomitance (mulabasa), and they place the prepositional phrase in the position of a state (hal) belonging to the pronoun of the object. Its weakness is evident.