Exodus 15:23-25

23When they came to Marah, they could not drink the water of Marah because it was bitter; therefore it was named Marah.

24And the people grumbled against Moses, saying, "What shall we drink?"

25And he cried to the Lord, and the Lord showed him a log, and he threw it into the water, and the water became sweet. There the Lord made for them a statute and a rule, and there he tested them,