Adrian Schoolcraft is the name that comes to mind for me: https://en.wikipedia.org/wiki/Adrian_Schoolcraft

> Schoolcraft amassed a set of tapes which demonstrated corruption and abuse within New York City's 81st Police Precinct. The tapes include conversations related to the issues of arrest quotas and investigations. [...] Schoolcraft was harassed, particularly in 2009, after he began to voice his concerns within the precinct. He was told he needed to increase arrest numbers and received a bad evaluation.

His fellow officers had him involuntarily committed to a psychiatric ward. They told the hospital that his claims were a sign of paranoid delusions. He was eventually vindicated, but his career was destroyed.