Is A Tribal Entity Allowed To Refuse Overtime Pay When Overtime Hours Are Worked?

I currently work at a tribal owned gas station in Washington State, USA. I'm considered part time, but working full-time hours. However I was just informed that if I work more than 40 hours in a work week, I do not qualify for overtime wages. Is this legal? References/sources would be much appreciated, I have scoured Google and cannot find a trusted source that explicitly says yes or no. I would like something to present to my manager as a "hey this is what the law says" type of thing. My manager has agreed to reduce my hours at my request as I'm not willing to work overtime without overtime wage. TYSM for any advice