Exodus 15:22-26

22So Moses brought Israel from the Red Sea; then they went out into the Wilderness of Shur. And they went three days in the wilderness and found no water.

23Now when they came to Marah, they could not drink the waters of Marah, for they were bitter. Therefore the name of it was called Marah.

24And the people complained against Moses, saying, “What shall we drink?”

25So he cried out to the Lord, and the Lord showed him a tree. When he cast it into the waters, the waters were made sweet. There He made a statute and an ordinance for them, and there He tested them,

26and said, “If you diligently heed the voice of the Lord your God and do what is right in His sight, give ear to His commandments and keep all His statutes, I will put none of the diseases on you which I have brought on the Egyptians. For I am the Lord who heals you.”