Exodus 15:22-26

Then Moses led Israel from the Red Sea, and they went out into the wilderness of Shur; and they went three days in the wilderness and found no water.

When they came to Marah, they could not drink the waters of Marah, because they were bitter; for that reason it was named Marah.

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

Then he cried out to the Lord, and the Lord showed him a tree; and he threw it into the waters, and the waters became sweet. There He made for them a statute and regulation, and there He tested them.

And He said, “If you will listen carefully to the voice of the Lord your God, and do what is right in His sight, and listen to His commandments, and keep all His statutes, I will put none of the diseases on you which I have put on the Egyptians; for I, the Lord, am your healer.”