Exodus 15:24

Moses led Israel onward 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 couldn't drink from the waters of Marah, for they were bitter. Therefore its name was called Marah.

Then he cried to Yahweh. Yahweh showed him a tree, and he threw it into the waters, and the waters were made sweet. There he made a statute and an ordinance for them, and there he tested them;

and he said, "If you will diligently listen to the voice of Yahweh your God, and will do that which is right in his eyes, and will pay attention 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 am Yahweh who heals you.