Source one — the supply side. Pressurised, and it will not stop on its own. A burst pipe, a failed fitting, a split flexible hose, a leaking valve, a fractured supply line. The water is under pressure and arriving continuously whether anyone is home or not. How to stop it: the fixture stop if you can identify which fixture is involved — the small valve on the supply line under a sink or behind a toilet. The main shutoff if you can't, if the leak is inside a wall or ceiling, or if the fixture stop won't hold. Then open a low tap — a ground-floor or basement one — to drain the pipes above the failure and reduce what continues to escape. This is the one where the main is the answer, and it's the reason everybody believes it's the answer to everything.