Exodus 15:23-25

Then they came to Marah, where there was water, but they could not drink it because it was too bitter. (That is why the place was named Marah.) The people grumbled to Moses and asked, “What will we drink?” So Moses cried out to the LORD, and the LORD showed him a tree. When Moses threw the tree into the water, the water became good to drink. There the LORD gave the people a rule and a law to live by, and there he tested their loyalty to him.