Exodus 15:24-25

24Then the people complained. They spoke against Moses. They said, ‘There is nothing for us to drink!’

25Then 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.