The hand with the knife (Grimm's Fairy Tale)

There once lived a girl who had three brothers. The boys meant everything to their mother, and the girl was always put at a disadvantage and treated badly. Every day she had to go out to a barren heath to dig peat, which was used for cooking and heating. A dull old tool was all she had for that nasty work.

The little girl had an admirer, an elf who lived in a hill near her mother's house. Whenever she passed by that hill, the elf would stretch his hand out of a boulder and hold out a very sharp knife that had special powers and could cut through anything. She was able to dig out the peat quickly with that knife, go home with the required amount, and when she got to the boulder, she tapped on it twice. The hand would then reach out to take back the knife.

When the mother began to notice how quickly and effortlessly the girl brouhgt home the peat, she told the brothers that someone else had to be helping her with the work, otherwise it wouldn't be possible. The brothers stealthily followed her and saw how she got the magical knife, then caught up with her and forced her to give it to them. They headed back, struck the rock as the girl had done, and when the good elf stretched his hand out, they cut it off with his own knife. The bloody arm pulled back, and because the elf believed that his beloved had betrayed him, he was never seen again.