Exodus 15:22-25

22And Moses brought Israel 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.

23And they came to Marah, and could not drink the waters of Marah, for they were bitter; therefore the name of it was called Marah.

24And the people murmured against Moses, saying, What shall we drink?

25And he cried to Jehovah; and Jehovah shewed him wood, and he cast it into the waters, and the waters became sweet. There he made for them a statute and an ordinance; and there he tested them.