Tafsir of Al-Hijr 15:9

{Indeed, it is We who have sent down the Reminder} This is a rebuttal to their denial and mockery in their saying: {And they said, "O you upon whom the Reminder has been sent down..."}.

For this reason, He said: "Indeed, it is We," emphasizing to them definitively and absolutely that He is the One who sent it down, and that He is the One who sent Gabriel with it to Muhammad (peace be upon him), with watchers before him and behind him, until it descended and was delivered protected from the devils.

{And indeed, We will be its guardian} He is its guardian at all times against any addition, omission, distortion, or alteration, unlike the previous scriptures. He did not take charge of preserving them; rather, He entrusted them to the rabbis and scholars, who then differed among themselves out of envy, leading to distortion. But He did not entrust the Quran to anyone other than Himself for its preservation.

If you ask: Since the saying {Indeed, it is We who have sent down the Reminder} is a rebuttal to their denial and mockery, how does it connect to His saying {And indeed, We will be its guardian}?

I say: He has made this [preservation] a proof that it is sent down from Him as a sign. For if it were the speech of humans or not a sign, it would be subject to addition and omission, just as every other speech is subject to them.

It is also said: The pronoun in {its guardian} refers to the Messenger of Allah (peace be upon him), as in the Almighty’s saying: {And Allah will protect you from the people} (Al-Ma'idah: 67).

{And We have already sent [messengers] before you among the sects of the former peoples. And no messenger came to them except that they were at him mocking.}