Exodus 15:23, 25

When they came to the oasis of Marah, the water was too bitter to drink. So they called the place Marah (which means “bitter”).

So Moses cried out to the LORD for help, and the LORD showed him a piece of wood. Moses threw it into the water, and this made the water good to drink. It was there at Marah that the LORD set before them the following decree as a standard to test their faithfulness to him.