There were two brothers; one was rich, the other poor. However, the rich one gave nothing to the poor one, who barely made a living as a grain dealer. Things often went so badly for him that he had no bread for his wife and children.
One day he was pushing his cart through the forest when off to the side he saw a large bare mountain. He had never seen it before, so he stopped and looked at it with amazement. While he was standing there he saw twelve tall wild men approaching. Thinking that they were robbers, he pushed his cart into the thicket, climbed up a tree, and waited to see what would happen.
The twelve men went to the mountain and cried out, "Mount Semsi, Mount Semsi, open up." The barren mountain immediately separated down the middle. The twelve men walked into it, and as soon as they were inside it shut.
A little while later it opened again, and the men came out carrying heavy sacks on their backs. As soon as they were all back in the daylight they said, "Mount Semsi, Mount Semsi, close." Then the mountain went back together, and the entrance could no longer be seen. Then the twelve men went away.
When they were completely out of sight, the poor man climbed down from the tree. He was curious to know what secret was hidden in the mountain, so he went up to it and said, "Mount Semsi, Mount Semsi, open up," and the mountain opened up for him as well.
He went inside, and the entire mountain was a cavern full of silver and gold, and in the back of the cavern there lay great piles of pearls and sparkling jewels, piled up like grain. The poor man did not know what he should do, whether or not he could take any of these treasures for himself. At last he filled his pockets with gold, but he left the pearls and precious stones lying where they were.
Upon leaving he too said, "Mount Semsi, Mount Semsi, close," and the mountain closed. Then he went home with his cart.
He no longer had any cares, for with his gold he could buy bread for his wife and children, and wine as well. He lived happily and honestly, gave to the poor, and did good for everyone. When he ran out of money he went to his brother, borrowed a bushel, and got some more money, but did not touch any of the very valuable things. When he wanted to get some more money for the third time he again borrowed the bushel from his brother. However, the rich man had long been envious of his brother's wealth and of the fine household that he had furnished for himself. He could not understand where the riches came from, and what his brother wanted with the bushel. Then he thought of a trap. He covered the bottom of the bushel with pitch, and when he got the bushel back a gold coin was sticking to it.
He at once went to his brother and asked him, "What have you been measuring in the bushel?"
"Wheat and barley," said the poor brother.
Then he showed him the gold coin and threatened that if he did not tell the truth he would bring charges against him before the court. Then the poor man then told him everything that had happened to him. The rich man immediately had his wagon hitched up and drove away, intending to do better than his brother had done, and to bring back with him quite different treasures.
When he came to the mountain he cried out, "Mount Semsi, Mount Semsi, open up."
The mountain opened, and he went inside. There lay the riches all before him, and for a long time he did not know what he should take hold of first. Finally took as many precious stones as he could carry. He wanted to carry his load outside, but as his heart and soul were entirely occupied with the treasures, he had forgotten the name of the mountain, and cried out, "Mount Simeli, Mount Simeli, open up."
But that was not the right name, and the mountain did not move, remaining closed instead. He became frightened, and the longer he thought about it the more he became confused, and all of the treasures were of no use to him.
In the evening the mountain opened up, and the twelve robbers came inside. When they saw him they laughed and cried out, "Bird, we have you at last. Did you think we did not notice that you came here twice? We could not catch you then, this third time you shall not get out again."
He cried out, "I wasn't the one. It was my brother!"
But however much he begged for his life, and in spite of everything that he said, they cut off his head.
Source: Jacob and Wilhelm Grimm, "Simeliberg," Kinder- und Hausmärchen gesammelt durch die Brüder Grimm (Children's and Household Tales -- Grimms' Fairy Tales), vol. 2, 7th ed. (Göttingen: Verlag der Dieterichschen Buchhandlung, 1857), no. 142, pp. 259-61.
The Grimms' source: Ludowine von Haxthausen (1795-1872), who heard the tale in the vicinity of Münster. The story is clearly a shortened version of "Ali Baba and the Forty Thieves" from the 1001 Nights.