I say:

The preposition bā’ can indicate accompaniment, as when you say, “He came with his masters and children.” Thus, He sent it down containing and bearing His knowledge. Whatever reports it contains are given with Allah’s knowledge, and whatever commands it contains are issued with Allah’s knowledge. This is unlike speech sent down from someone other than Allah. Such speech may be false and unjust, like Musaylimah’s Quran. Or it may be true, yet contain only the knowledge of the created being who uttered it. It indicates Allah Most High’s knowledge only by implication: namely, that Allah knows whatever is true.

The Quran, however, contains Allah’s knowledge directly. He sent it down with His knowledge, not anyone else’s; nor is it speech devoid of knowledge. Since He sent it down with His knowledge, it follows that it is truth from Allah, and that the Messenger is indeed a messenger from Allah, who has made His knowledge clear within it.

Al-Zajjāj said:

A “witness” is one who makes clear what he testifies to. Allah makes that clear and also knows it to be true.

(I say:

Regarding His words, “But Allah bears witness,” His testimony consists in His clarification and disclosure, His demonstration and declaration. The clear signs through which He has demonstrated the Messenger’s truthfulness attest to it. Among them is the Quran, which is testimony in words. It is itself a sign and a miracle attesting to his truthfulness, just as the other signs do. All these signs are testimony from Allah, just like testimony in words, and may be even more compelling. This is why He mentioned this in Sūrat Hūd when He challenged them to produce its like, saying: