Exodus 15:22-25

22Then Moses led Israel from the Red Sea and they went into the Desert of Shur. For three days they traveled in the desert without finding water.

23When they came to Marah, they could not drink its water because it was bitter. (That is why the place is called Marah.)

24So the people grumbled against Moses, saying, “What are we to drink?”

25Then Moses cried out to the Lord, and the Lord showed him a piece of wood. He threw it into the water, and the water became fit to drink. There the Lord issued a ruling and instruction for them and put them to the test.