(Note: I wasted too much time getting the quotes, so I'm late, but I'm posting this anyway because it contains the quotes.)
1969: Larry Niven's "Not Long Before the End," The Magazine of Fantasy and Science Fiction, April 1969. It was republished in the collection The Magic May Return (1981), which is where I read it.
The Warlock hints that he has discovered a terrible truth:
The Warlock had found his terrible truth in middle age.
By that time he had traveled widely. It was not from choice. It was simply that he was a powerful magician, and he used his power, and he needed friends.
He knew spells to make people love a magician. The Warlock had tried these, but he did not like the side effects. So he commonly used his great power to help those around him, that they might love him without coercion.
He found that when he had been ten to fifteen years in a place, using his magic as whim dictated, his powers would weaken. If he moved away, they returned. Twice he had had to move, and twice he had settled in a new land, learned new customs, made new friends. It happened a third time, and he prepared to move again. But something set him to wondering.
Why should a man's powers be so unfairly drained out of him?
It happened to nations too. Throughout history, those lands which had been richest in magic had been overrun by barbarians carrying swords and clubs. It was a sad truth, and one that did not bear thinking about, but the Warlock's curiosity was strong.
So he wondered, and he stayed to perform certain experiments.
His last experiment involved a simple kinetic sorcery set to spin a metal disc in midair. And when that magic was done, he knew a truth he could never forget.
And then he is forced to use it, and he explains:
Hap clutched his messily severed wrist, and he said, "But what happened?"
"Mana," the Warlock mumbled. He spat out a complete set of blackened teeth. "Mana. What I discovered was that the power behind magic is a natural resource, like the fertility of the soil. When you use it up, it's gone."
For some reason "he spat out a complete set of blackened teeth" is one of those body horror images that has always stuck with me.