Exodus 15:23-25

And when they came to Marah, they could not drink the water there because it was bitter. (That is why it was named Marah.) So the people grumbled against Moses, saying, “What are we to drink?” And Moses cried out to the Lord, and the Lord showed him a log. And when he cast it into the waters, they were sweetened. There the Lord made for them a statute and an ordinance, and there He tested them,