Exodus 15:23-25

They came to Marah, but they could not drink the water at Marah because it was bitter — that is why it was named Marah.  The people grumbled to Moses, “What are we going to drink? ”  So he cried out to the LORD, and the LORD showed him a tree. When he threw it into the water, the water became drinkable. The LORD made a statute and ordinance for them at Marah, and he tested them there.