Exodus 15:23-25

When they came to Marah, they could not drink its waters for they were bitter; therefore it was named Marah [bitterness]. The people murmured against Moses, saying, What shall we drink? And he cried to the Lord, and the Lord showed him a tree which he cast into the waters, and the waters were made sweet. There [the Lord] made for them a statute and an ordinance, and there He proved them