They said, "If only he would bring us a sign from his Lord!" Did not proof come to them from what is in the previous book?
Translation endnotes
[20:133] The truth-value of the Quran's message and its historical dimension provide evidence for its divine nature. The occurrence of the singular word bayyina (evidence/proof) in the Quran being exactly 19 times and the existence of the code 19 in previous scriptures is another fulfillment of this verse. See 46:10.