Either the "P" trap (called that because it's shaped like the letter "P", laying down) or the valve assembly. You'll need to open the access panel to figure it out. If the builder included an access panel it will be on the the other side of the wall where the water valves are. If the builder didn't put in an access panel, you may need to make your own.
ANS 2 -You may even have to open the ceiling carefully to find this leak. -I've done this a few times, and as long as you have a wallboard style ceiling it's not too hard. -Use a studfinder to accurately find joists around the leak, then cut halfway across the joists with a craft knife to remove a section of the wallboard. I usually remove about 16" x 20 " directly under the drain end of the tub. -This gives adequate room to find and fix the leak. (last time it was a thin brass P trap that had rotted out) Fix leak, then replace wallboard with new.