Exodus 15:22-25

22Then Moses made Israel set out from the Red Sea, and they went into the wilderness of Shur. They went three days in the wilderness and found no water.

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,