Exodus 15:23-27

Then they came to Marah, but they could not drink its waters because they were bitter; therefore it was named Marah (bitter). The people [grew discontented and] grumbled at Moses, saying, “What are we going to drink?” Then he cried to the LORD [for help], and the LORD showed him a tree, [a branch of] which he threw into the waters, and the waters became sweet. T here the LORD made a statute and an ordinance for them, and there He tested them, saying, “If you will diligently listen and pay attention to the voice of the LORD your God, and do what is right in His sight, and listen to His commandments, and keep [foremost in your thoughts and actively obey] all His precepts and statutes, then I will not put on you any of the diseases which I have put on the Egyptians; for I am the LORD who heals you.” Then the children of Israel came to Elim where there were twelve springs of water and seventy date palms, and they camped there beside the waters.