Celtics guard Isaiah Thomas has been fined for using inappropriate language in an exchange with a Wizards fan after Boston lost in Game 4 at Verizon Center. 

The NBA began investigating the incident after a fan posted a video of the incident on Instagram.

In the video, Thomas was heard telling a person behind the Celtics bench that, "I will f--- you up and you know that."

The contentious series between the Wizards and Celtics shifts back to Boston for Game 5 tonight. 

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