Exodus 15:25-26

25So Moses cried out to the Lord for help, and the Lord showed him a piece of wood. Moses threw it into the water, and this made the water good to drink. It was there at Marah that the Lord set before them the following decree as a standard to test their faithfulness to him.

26He said, “If you will listen carefully to the voice of the Lord your God and do what is right in his sight, obeying his commands and keeping all his decrees, then I will not make you suffer any of the diseases I sent on the Egyptians; for I am the Lord who heals you.”