There is a debate in the gemara (Ketuvot 10a) whether the ketuvah is mi’di’orraita, Biblical, or mi’di’rabanan, rabbinic. The position that ketuvah is Biblically-based is an individual one, that of Rabban Shimon ben Gamliel, and he even states this position somewhat tentatively – mikan samkhu li’ktuvat isha min haTorah, from here the Rabbis found support to the institution of the ketuvah from the Torah. It would seem that the position that ketuvah is from the Torah is somewhat tentative, and indeed, most Rishonim conclude, based on this and other sugyot, that the halakha is not like Rabban Shimon ben Gamliel