Exodus 15:22-26 NIV
Then 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. 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. He said, “If you listen carefully to the LORD your God and do what is right in his eyes, if you pay attention to his commands and keep all his decrees, I will not bring on you any of the diseases I brought on the Egyptians, for I am the LORD, who heals you.”
NIV: New International Version