Leaking shower pan or leaking pipe .
Leaking pipe: cut away ceiling and have a look...

If it's not the pipe, it's the shower pan:
There is a plastic or lead pan under the shower that goes up the walls a bit.
We installed a few shower pans, ....I was going to write a how-to webpage but haven't.

Use jackhammer to tear out tile and mud (sand and concrete mix) floor so the drain shows, and bare curb shows ... usually tears up pretty easy.
Remove tile wall up about 6-12"
Might be able to reuse old drain and save a lot of work, but the mud has to be cleaned off.
Strip wall down to 2x4 studs
Lay the new pan as shown in photo ... the drain has 4 bolts that clamp down on the pan, forming a seal that keeps water from leaking outside the shower but lets the pan drain water into the drain
Use durrock or concrete board to fill in the walls and cover the curb.
I usually slope the curb a bit toward the drain and not out onto the bathroom floor.

New shower floor: buy sand mix and add just a tad bit of water so the mix is damp but NOT runny ... you want it to be dry enough that you can make a ball out of it using your hand, but the ball will fall apart once out of your hand ... if it's runny, the water will seep down into drain carrying cement that will dry inside the drain ... meaning you have to tear out everything and start over.. or replace pipe from underneath...

Lay the sand mix in a gentle slope toward the drain ... using different lengths of 2x4 to level and pound the sand mix smooth.
While the sand mix is still damp, sprinkle dry thin set over the surface and lay the tile floor, or wait til following day and apply tile using thin set or epoxy etc.
Then apply tile on wall. It's hard heavy work on your knees, which is why I migrated away from it after a few years.
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