Proofs that the "Book" (Kitab) that the Jews had in Arabia wasn't the modern-day Bible:

 

-  Gospel of Jesus.

-  Nag Hammadi Library that was found in Egypt.

-  Not all of the so-called Gnostics were the same.

-  The Mosaic Law is the Jews' Kitaab, and is Moses' Thikr.

-  The Jews fled the persecution of Rome in Northen Arabia to Madina.  They had their own writings with them that were not part of the writtings that were with the Romans.

-  The Jews' Holy Scriptures are mixed between the books that are in the Old Testament and the Talmud and other writtings.  They're no ONLY limited to the OT.

-  No one knows what the Jews of Arabia had from writings.  And they certainly would've hid the ones that prove and prophecy about Prophet Muhammad.

   -  The OT calls them liars and prostitutes.

-  There is a narration about Prophet Muhammad, calling the writting that was brought to him as Holy and Divine.  This is not from the Glorious Quran, and hence, it is doubtful and could be a big lie that was invented on the Prophet.  But even if we assume that it was true, the Prophet called the Law of punishing the adulterer as Holy in their writings.

-  The original canon was lost according to the Christians themselves.

-  The Glorious Quran calls the Christians' beliefs as nothing but conjecture.

-  The Glorious Quran Says that the Jews alter (yu7arrifun) the Words.

-  In the OT, we have a lot of evidence for Islam:

    -  The believers' title is Muslims.
    -  The Prophet of Arabia Prophecies.
    -  Mecca and the Path of Holiness.

-  IMPORTANT:  The Holy Quran recognizes, in several places, and is ok with the Kitaab of the Jews being not in its pure form, and containing alterations.  This still doesn't take away the core Divine Message and Moral Code of the Scriptures, even if they were corrupt.  The Core Divine Message and the Moral Code are well preserved.

    -  The Glorious Quran corrected the previous scriptures and showed where they were in error, and what the Truth was.  Ex: Jesus' crucifixion, Haman story, Jesus spoke when he was a baby, Mary sister of Aaron quote of the Jews who said it,