Exodus 15:22-26

Moses led the Israelites away from the Red Sea into the Desert of Shur. They traveled for three days in the desert but found no water. Then they came to Marah, where there was water, but they could not drink it because it was too bitter. (That is why the place was named Marah.) The people grumbled to Moses and asked, “What will we drink?” So Moses cried out to the LORD, and the LORD showed him a tree. When Moses threw the tree into the water, the water became good to drink. There the LORD gave the people a rule and a law to live by, and there he tested their loyalty to him. He said, “You must obey the LORD your God and do what he says is right. If you obey all his commands and keep his rules, I will not bring on you any of the sicknesses I brought on the Egyptians. I am the LORD who heals you.”