Exodus 15:23-25

When they came to Marah, they could not drink the water of Marah because it was bitter. That is why it was called Marah. And the people complained against Moses, saying, “What shall we drink?” He cried out to the Lord; and the Lord showed him a piece of wood; he threw it into the water, and the water became sweet. There the Lord made for them a statute and an ordinance and there he put them to the test.