Exodus 15:23-25

23And when they came to Marah, they could not drink of the waters of Marah, for they were bitter: therefore the name of it was called Marah.

24And the people murmured against Moses, saying, What shall we drink?

25And he cried unto the LORD; and the LORD shewed him a tree, and he cast it into the waters, and the waters were made sweet. There he made for them a statute and an ordinance, and there he proved them;