Tell them, (O Prophet): "Then do bring a Book from Allah which is a better guide than either of them, and I will follow it!1 Do so if you are truthful!"
Footnotes
[1]
That is, “I have to follow the guidance in any case, provided that it is not forged but is real guidance from God. If you possess a divine Book which gives better guidance than the Quran and the Torah, you should produce it: I shall follow it without any hesitation.”
[2]
A grammatical shift from 1st person to 3rd person.