Then the young lady begged him to teach her how to put a lady to sleep. He knew what she was thinking, but even so he taught her this and other things besides. For God our Lord wished it so, and he taught her three names that she should write down, and every time he wanted to lie with her, they would have such great power that, for as long as she had them on her person, no man could know her carnally.

Vulgate Merlin