Exodus 15:23-25

And they arrived at Marah. They were unable to drink the waters of Marah because they were bitter. Therefore, he also established a name befitting the place, calling it 'Marah,' that is, bitterness. And the people murmured against Moses, saying: "What shall we drink?" So he cried out to the Lord, who showed him a tree. And when he had cast it into the waters, they were turned into sweetness. In that place, he established instructions for him, and also judgments. And he tested him there