Exodus 15:23-25

Then they came to Marah, but they could not drink its waters because they were bitter; therefore it was named Marah (bitter). The people [grew discontented and] grumbled at Moses, saying, “What are we going to drink?” Then he cried to the LORD [for help], and the LORD showed him a tree, [a branch of] which he threw into the waters, and the waters became sweet. T here the LORD made a statute and an ordinance for them, and there He tested them