Exodus 15:23-25

When they came to Marah, they couldn’t drink from the waters of Marah, for they were bitter. Therefore its name was called Marah. The people murmured against Moses, saying, “What shall we drink?” Then he cried to the LORD. The LORD showed him a tree, and he threw it into the waters, and the waters were made sweet. There he made a statute and an ordinance for them, and there he tested them.