wellspring

The source of water for a stream, spring or well; a fountainhead; a wellhead.

A perennial source of anything; a fountainhead of supply or emanation; resource.figuratively

• Krapp’s Last Tape has been acted by prisoners in San Quentin and made into a chamber opera. It goes to the wellspring of an actor. Well, most of the time: when Albert Finney played the part, Beckett went to sleep in rehearsals.