Exodus 15:23-25

When they came to Marah, they could not drink the waters of Marah, for they were bitter; therefore it was named Marah. So the people grumbled at Moses, saying, “What shall we drink?” Then he cried out to the Lord, and the Lord showed him a tree; and he threw it into the waters, and the waters became sweet. There He made for them a statute and regulation, and there He tested them.