Exodus 15:22-26

Then Moses led the people of Israel away from the Red Sea, and they moved out into the desert of Shur. They traveled in this desert for three days without finding any water. When they came to the oasis of Marah, the water was too bitter to drink. So they called the place Marah (which means “bitter”). Then the people complained and turned against Moses. “What are we going to drink?” they demanded. 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. He said, “If you will listen carefully to the voice of the LORD your God and do what is right in his sight, obeying his commands and keeping all his decrees, then I will not make you suffer any of the diseases I sent on the Egyptians; for I am the LORD who heals you.”