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.1 24And the people murmured against Moses, saying, What shall we drink? 25And he cried unto Yhwh; And Yhwh shewed him a tree, which when he had cast into the waters, the waters were made sweet: there he made for them a statute and an ordinance, and there he proved them,