Basically what Radarspod said.
You can't have X aeroplanes all shouting continuous mode C interrogations at one another without trashing the SSR frequencies.
One of the main challenges of TCAS from a technical point of view is to minimise the ammount of SSR traffic. The answer to this is a number of techniques one of which is the use of a roll call and selective addressing so Mode S is an essential building block of TCAS.
As far as a reference. Well, a quick google will show you any number of installations that have TCAS and Mode S, and plent yof references that discuss the transponder arcitecture and data formats. But of course that doesn't prove the non existance of a TCAS that works using mode C...
My basis for saying this is having done an original manufacturers course and having been the lead instructor on this system at a major airline. Everyone else here agrees with me, so no offence but I think the onus is on you to provide a credible reference that a TCAS II installation can be embodied without a Mode S transponder.
pb