Eureka, I found it:
http://www.eurocontrol.int/acas/webd..._27Jul2002.doc
quote: "Your attention is specifically invited to paragraph 3.2.3b), item 12), which states: “if pilots simultaneously receive instructions to manoeuvre from ATC and an RA which are in conflict, the pilot should follow the RA”. The reason for this statement is that ATC may not be aware of ACAS RAs, and may therefore issue instructions that are contrary to an ACAS RA."