Exodus 15:23-25

Then they came to a place called Marah, but the water there was so bitter that they could not drink it. That is why it was named Marah. The people complained to Moses and asked, “What are we going to drink?” Moses prayed earnestly to the LORD, and the LORD showed him a piece of wood, which he threw into the water; and the water became fit to drink. There the LORD gave them laws to live by, and there he also tested them.