Exodus 15:24

22 Then Moses led Israel from the Red Sea, and they went out into the Desert of Shur. For three days they walked in the desert without finding water.

23 And when they came to Marah, they could not drink the water there because it was bitter. (That is why it was named Marah.)

24 So the people grumbled against Moses, saying, “What are we to drink?”

25 And Moses cried out to the LORD, and the LORD showed him a log. And when he cast it into the waters, they were sweetened. There the LORD made for them a statute and an ordinance, and there He tested them,

26 saying, “If you will listen carefully to the voice of the LORD your God, and do what is right in His eyes, and pay attention to His commands, and keep all His statutes, then I will not bring on you any of the diseases I inflicted on the Egyptians. For I am the LORD who heals you.”