Exodus 15:23-25

When they came to Marah, they could not drink its water because it was bitter. (That is why the place is called Marah.) So the people grumbled against Moses, saying, “What are we to drink?” Then 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.