How to say no to your senior when he constantly pawns of work to you?

I have been in the loop of constantly getting more and more work every day and am doing more work than people in my team at the same level. Every day, I try to get my work done on time, but I am getting more work just an hour or 2 before the time to log out, and am unable to figure out a way to say no or decline in a polite way. What to do?