Exodus 15:23-25

15:23 Then 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.)

15:24 So the people murmured against Moses, saying, “What can we drink?” 15:25 He 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.