Exodus 15:23-25

23Then they came to Marah, but they were not able to drink the waters of Marah, because they were bitter. (That is why its name was Marah.)

24So the people murmured against Moses, saying, “What can we drink?”

25He cried out to the Lord, and the Lord showed him a tree. When Moses threw it into the water, the water became safe to drink. There the Lord made for them a binding ordinance, and there he tested them.