Al-Ahqaf: 10
"Say, 'Have you considered...'"
The apodosis (the result of the conditional clause) is omitted. The implied meaning is: "If the Qur'an is from Allah and you have disbelieved in it, are you not wrongdoers?" This omission is indicated by His saying: "Indeed, Allah does not guide the wrongdoing people."
The "witness from the Children of Israel" is ‘Abdullah ibn Salam. When the Messenger of Allah (ﷺ) arrived in Medina, he looked at his face and knew it was not the face of a liar. He contemplated him and realized he was the awaited Prophet. He asked him:
"I am going to ask you about three things that only a Prophet knows: What is the first of the signs of the Hour? What is the first food the people of Paradise will eat? And why does a child resemble his father or his mother?"
The Prophet (ﷺ) replied: "As for the first sign of the Hour, it is a fire that will gather them from the East to the West. As for the first food of the people of Paradise, it is the extra lobe of a whale's liver. As for the child, if the man's fluid precedes the woman's, the child resembles him; if the woman's fluid precedes, the child resembles her."
‘Abdullah said: "I bear witness that you are truly the Messenger of Allah." Then he said: "O Messenger of Allah, the Jews are a people of slander. If they learn that you have asked them about me, they will slander me before you." When the Jews came, the Prophet (ﷺ) asked them: "What kind of man is ‘Abdullah among you?" They replied: "He is our best and the son of our best, our master and the son of our master, our most learned and the son of our most learned." He said: "What if ‘Abdullah were to accept Islam?" They said: "May Allah protect him from that!" Then ‘Abdullah came out to them and said: "I bear witness that there is no god but Allah and that Muhammad is the Messenger of Allah." They said: "He is our worst and the son of our worst," and they disparaged him. He said: "This is what I feared, O Messenger of Allah."
Sa‘d ibn Abi Waqqas said: "I never heard the Messenger of Allah (ﷺ) say of anyone walking on the face of the earth that he is from the people of Paradise, except for ‘Abdullah ibn Salam." It is regarding him that the verse was revealed: "And a witness from the Children of Israel has testified to the like of it."
The pronoun refers to the Qur'an. That is, "to the like of it in meaning," which is what is found in the Torah regarding the meanings that correspond to the meanings of the Qur'an, such as monotheism, promises, and warnings. This is supported by His saying: "And indeed, it is in the scriptures of the former peoples" (Ash-Shu'ara: 196), "Indeed, this is in the former scriptures" (Al-A'la: 18), and "Likewise, it is revealed to you and to those before you" (Ash-Shura: 3).
It is also possible the meaning is: "If it is from Allah and you disbelieved in it, and a witness testified to that—meaning its being from Allah..."
If you ask: "Inform me about the syntax of this speech so I may understand its meaning through its structure."
I say: The first waw (and) conjoins "you disbelieved" to the conditional verb, just as thumma (then) conjoins it in His saying: "Say, 'Have you considered if it is from Allah, then you disbelieved in it?'" (Fussilat: 52). Likewise, the final waw conjoins "you grew arrogant" to "a witness testified." As for the waw in "and a witness testified," it conjoins the sentence "A witness from the Children of Israel testified to the like of it, and he believed" to the sentence "If it is from Allah and you disbelieved in it."
The meaning is: "Say, 'Inform me, if the fact that the Qur'an is from Allah is combined with your disbelief in it, and the testimony of the most learned of the Children of Israel to the descent of its like and his belief in it is combined with your arrogance toward it and toward believing in it—are you not the most astray and most unjust of people?'"
He made the belief in his saying "and he believed" a result of the testimony to "the like of it," because when he knew that its like was revealed to Moses (peace be upon him), and that it was of the same nature as revelation and not the speech of humans, he was fair to himself, testified to it, and acknowledged it; thus, belief was the result of that.