Folks will point you to the hands and handlebars as the offending spot. But you must think of your nervous system as a system, and consider the full anatomy before you eliminate potential sources of this issue. The nerve root for these is at the C8/T1 area of the spine. You may be tensing or doing something that is impinging anywhere along the route from to the fingertips. Posture matters, as does weight balance, as does core strength, as does hand placement.

I'm guessing that you don't "lean" into your hands when you ride, right? You work to keep a light touch on the bars?