intellegent: i dont agree with Give Me a Break, but he specifically said all he wants to know is where the makor mideoraiysa is. and so far no one here has been able to answer him legitimately. siting meforshim, medroshim, and speeches by rabbi weiss is all nice, but not legitimate. he is asking how do we know we have a chiyuv, and that it is not just a nice thing. pashuta yid is the only one who has answered on the very basic terms of tznius, al pi ervoh.
the real answer is that it is from Micha. the pasuk says “hatzne’a leches im elokecha”, which Rov Elazar Bar Tzadok in meseches sukkah (daf mem tes amud bays)connects to tznius in both dress and charachter. the passuk in micha is perek vuv pasuk ches.
now i am no lamdon, but 3 minutes googling got me this information. this is the entire makor, and that technically makes tznius not one of the 613 mitzvos (besides the fact that it is not listed in any of the lists of the 613 mitzvos that we have from different rishonim).
now that we go that out of the way, everyone can argue!!