From Figures of Earth (1921) by James Branch Cabell:
He came upon Manuel like a whirlwind, and Manuel had no choice in the matter. So they fought, and presently Manuel brought the vermilion knight to the ground, and, dismounting, killed him. It was noticeable that from the death-wound came no blood, but only a flowing of very fine black sand, out of which scrambled and hastily scampered away a small vermilion-colored mouse.
From Adept's Gambit (1947) by Fritz Leiber:
Had any of them been watching the corpse of Anra Devadoris at this moment, they would have seen a slight twitching of the lower jaw. At last the mouth came open, and out leaped a tiny black mouse ... A wine jar, hurled by Fafhrd, shattered on the crack into which it shot; Fafhrd had seen, or thought he had seen, the untoward place from which the mouse had come.
"Mice in his mouth," he hiccuped. "What dirty habits for a pleasant young man! A nasty, degrading business, this thinking oneself an adept."
"I am reminded," said the Mouser, "of what a witch told me about adepts. She said that, if an adept chances to die, his soul is reincarnated in a mouse. If, as a mouse, he managed to kill a rat, his soul passes over into a rat. As a rat, he must kill a cat; as a cat, a wolf; as a wolf, a panther; and, as a panther, a man. Then he can recommence his adeptry. Of course, it seldom happens that anyone gets all the way through the sequence and in any case it takes a very long time. Trying to kill a rat is enough to satisfy a mouse with mousedom."
So, I am curious: was Leiber inspired by a passing phrase of Cabell’s, and expanded on the idea of what might happen if one killed an adept, or is there some piece of actual mythology—or simply an earlier author—that both of them might have been familiar with? Have any other authors played with this notion?