I had something similar last month 1/2” elbow behind the water heater spraying, plumber drained and removed water heater then replaced the elbow, re installed water heater, total cost was $370 same day service.  Bought house new in 1990 and this is the third elbow that’s started spraying since then, usually happens after the city has water off for whatever reason.
Keep a couple small 3/4” stainless hose clamps around and a piece of rubber such as a motorcycle inner tube , you can make a temporary patch to stop the leak and turn water back on if you want to take bids to get it repaired next time.