Exodus 15
23 Ref And when they came to Marah, the water was no good for drinking, for the waters of Marah were bitter, which is why it was named Marah.
24 Ref And the people, crying out against Moses, said, What are we to have for drink?
25 Ref And in answer to his prayer, the Lord made him see a tree, and when he put it into the water, the water was made sweet. There he gave them a law and an order, testing them;