Exodus 15:25-26

25Moses cried out to the Lord, and the Lord showed  him a piece of wood. He threw it into the water, and the water became sweet. There the Lord set down laws and rules for them to live by, and there he tested them.

26He said, “If you will listen carefully to the Lord your God and do what he considers right, if you pay attention to his commands and obey all his laws, I will never make you suffer any of the diseases I made the Egyptians suffer, because I am the Lord, who heals you.”