I don't think there is one. our MATS only provides examples for the situation where a TCAS climb or descent is commanded. Implies that (normally) you don't have to tell ATC. The exception would be if a controller had previously stated a min or max climb or descent rate, and TCAS countermanded that due to excessive closure. I would hope that would be highly unlikely; a controller would not normally ask for a vertical speed that would cause that situation. If it did happen, I'd suggest "### this is ###, unable to maintain xxxfpm due to TCAS" (while or immediately following the response to the prompt.)
Perhaps someone with more knowledge can provide the correct phrase, if applicable.