The FAQ mentions the following location: "Near Karmak's island and just a couple castles east of Cheshire is a strange area where there a three mountains arranged in a triangle in the middle of a forest. Search in the center of these mountains, and you will find a spring"

The spring that is surrounded by three mountains is the "holy spring". If you talk to the priest in the town to the east of it he will tell you to purify yourself in the spring. You must then go to the town south of the spring and talk to the mayor, he will ask you if you've heard of the spring, say "Yes", he will then tell you were it is located. Now that you know were it is send a class 4 priest to the spring and search, you will be asked if you wish to purify yourself, answer "Yes". Now travel to the town east of the spring and once again talk to the priest, he will teach you the spell of "Rebirth".