In Surah Āl ʿImrān, He says:

“Allah—there is no deity worthy of worship except Him, the Ever-Living, the Sustainer of all. He has sent down to you the Book in truth, confirming what came before it, and He sent down the Torah and the Gospel previously, as guidance for mankind, and He sent down the Criterion.”

Thus, He mentions His oneness first, then belief in what the messengers brought second. He mentions that He sent down the Book and the Criterion, just as He says:

“And when We gave Moses the Book and the Criterion.”

The term “Criterion” encompasses whatever distinguishes truth from falsehood, such as the signs with which the prophets were sent: the serpent, the white hand, and the parting of the sea. The Quran is a criterion in this respect: it is a tremendous sign of the prophethood of Muhammad—may Allah bless him and grant him peace—and a source of great knowledge. It is also a criterion in that, through its explanations, it distinguishes truth from falsehood, as He says:

“Blessed is He who sent down the Criterion upon His servant.”

For this reason, a number of scholars have interpreted “the Criterion” here as referring to the Quran. The term “Criterion” also encompasses Allah’s granting victory to His prophets and His believing servants, and His destruction of their enemies. Through this, He distinguishes His allies from His enemies; this, too, is among the signs. Allah Most High says:

“If you believe in Allah and in what We sent down upon Our servant on the Day of the Criterion—the day the two armies met.”

The signs that Allah makes evidence of the prophets’ truthfulness are among the things He sends down, as He says:

“They say, ‘Why has no sign been sent down to him from his Lord?’ Say, ‘Allah is certainly able to send down a sign.’”

And He says:

“If We willed, We could send down to them a sign from heaven before which their necks would remain bowed in submission.”

And Allah Most High says:

“But those who did wrong substituted words other than those they had been told, so We sent down upon the wrongdoers a punishment from heaven because of their disobedience.”

A fuller discussion of this belongs elsewhere.