Exodus 15:23-25

When they came to Marah, they could not drink from the waters because they were bitter. On account of this it was called Marah. So the people complained to Moses saying, “What are we going to drink?” So he cried out to ADONAI, and ADONAI showed him a tree. When he threw it into the waters, they were made sweet. There He made a statute and an ordinance for them, and there He tested them.