Exodus 15:23-25

Then they came to Marah. There was water at Marah, but it was too bitter to drink. (That is why the place was named Marah.) The people began complaining to Moses. They said, “Now what will we drink?” So Moses called to the LORD, and the , and the LORD showed him a large piece of wood. When Moses put the wood in the water, the water became good to drink. There the LORD put in place a law and a command for him and tested him to see if he would obey.