وَشَهِدَ شَاهِدٌ مِّنۢ بَنِىٓ إِسْرَٰٓءِيلَ عَلَىٰ مِثْلِهِۦ فَـَٔامَنَ وَٱسْتَكْبَرْتُمْ
"A witness from the Children of Israel bore witness to the like of it and believed; but you were arrogant."
The verse gives the second answer to the demand for "an earlier Book" in the fourth verse: the testimony of someone who knows that Book.
Who the witness is is not said in the verse. Various names are transmitted in the classical commentaries; I do not pass these on as certain knowledge. Since the verse itself gives no name, I record that the verdict is tied not to a name but to a stance: fa-āmana wa'stakbartum — one believed, the others grew proud.
ٱسْتَكْبَرْتُمْ — Form X: to see oneself as great, to lay claim to greatness. Its opposite is not faith but surrender. The verse ties the ground of the refusal not to a lack of knowledge but to a stance — and read together with the fourth verse of the sūra this is a consistency: when evidence comes to the side that demanded evidence, what appears is not a discussion of evidence.