Exodus 15:23-25

When they arrived at Marah, the water there was too bitter to drink. (That's why the place is called Marah.) So the people complained to Moses, asking, “What are we going to drink?” Moses called out to the Lord for help, and the Lord showed him a piece of wood. When he threw it into the water, it became sweet. There the Lord gave them rules and instructions and also tested their loyalty to him.