Exodus 15:23-25

²³When they came to Marah, they couldn`t drink from the waters of Marah, for they were bitter. Therefore the name of it was called Marah. ²⁴The people murmured against Moses, saying, "What shall we drink?" ²⁵Then he cried to Yahweh. Yahweh 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;