I found the leak but don't know how to fix it...

A few years ago, I had put a shower/tub in my bathroom downstairs.

A few days ago, I noticed in the laundry room, which is behind my bathroom, the wallboard by my pipes for the shower/bath was stained due to water leakage.

I took out the wallboard, then went into my bathroom and turned the water on the in the tub and saw no leak. Then I turned the shower on and returned to the laundry room to look at the pipes and I had a small Niagra fall going on. Where the faucet connects to the copper pipes is where the leak is.

The kind of faucet I have is a one-handle-and then a smaller handle to turn the shower on.

Any ideas on how to fix?

Thanks!!

I turned the shower on and returned to the laundry room to look at the pipes and I had a small Niagra fall going on. Where the faucet connects to the copper pipes is where the leak is.
You're making me guess where the leak is. It's not where the supplies connect to the valve or it would be leaking all the time. The only other copper pipe that I know of is the shower raiser. Repost your question with more details. Explain exactly where the leak is and the section of the valve that it's affecting. Regards, Tom