Exodus 15:23-25

And 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. And the people murmured against Moses, saying, What shall we drink? And he cried unto Jehovah; and Jehovah showed 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