Exodus 15:23-25

They did find water at Marah, but it was bitter, which is how that place got its name. The people complained and said, “Moses, what are we going to drink?” Moses asked the LORD for help, and the LORD told him to throw a piece of wood into the water. Moses did so, and the water became fit to drink. At Marah the LORD tested his people and also gave them some laws and teachings.