Celtic Warrior Reference?

[qos]

This evening at a group meeting, one person referenced a story she had heard from a professor, which she believed had been taken from Celtic lore: that when returning from battle, warriors stopped to bathe at each of seven pools, each of which was cooler than the one before it. The idea was that by the time they reached home, their battle rage had been cooled.

Does anyone have an actual reference for this story?