Bill Ackman's hedge fund had a brutal month

Pershing Square Capital, the $20 billion fund run by activist investor Bill Ackman, got crushed in September.

The fund fell 12.5% during the month. It's now down 12.6% for the first nine months of this year, according to a performance update.

Ackman's returns were largely dragged down by the fund's large stake in Valeant Pharmaceuticals.

The Canadian pharmaceutical company's stock crashed this week after all of the Democrats on the House Oversight and Government Reform Committee sent a letter to the committee's chairman, Jason Chaffetz (R-UT), asking him to subpoena the pharmaceutical company for documents related to price increases of acquired drugs.

Shares of Valeant collapsed fell more than 22% in September.

In mid-March, Ackman disclosed that his fund bought 19,473,933 shares, or a approximately 5.7% stake. Ackman is the third largest shareholder.

Last year, Pershing Square was the best-performing hedge fund, returning 40.4%.