Exodus 15:23-25

And when they came to Marah, they could not drink of the waters of Marah because it was bitter. Therefore the name of it was called Marah. And the people murmured against Moses, saying, “What shall we drink?” And he cried to the LORD. And the LORD showed him a tree. And when he had cast it into the waters, the waters were made sweet. There He made a decree and a law for them, and there He proved them.