Exodus 15:23-25

Then they arrived at Marah. But they could not drink the water there because it was too bitter. That is why they called that place ‘Marah’. Then the people complained. They spoke against Moses. They said, ‘There is nothing for us to drink!’ Then Moses called to the LORD for help. The LORD showed Moses a special tree. Moses threw a branch into the water. Then the water became good for them to drink. There at Marah, the LORD made a law for the Israelites to obey. He tested them there, to see if they would trust him.