Exodus 15:22-25

22Then Moses led Israel onward from the Sea of Reeds. They went out into the wilderness of Shur. But they travelled three days in the wilderness and found no water.

23When they came to Marah, they could not drink from the waters because they were bitter. On account of this it was called Marah.

24So the people complained to Moses saying, “What are we going to drink?”

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