Exodus 15:23-27

Now 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. And the people complained against Moses, saying, “What shall we drink?” So 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, and 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.” Then they came to Elim, where there were twelve wells of water and seventy palm trees; so they camped there by the waters.