Exodus 15:22-25

22 Moses led Israel onward from the Red Sea, and they went out into the wilderness of Shur; and they went three days in the wilderness, and found no water.  23 When 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.  24 The people murmured against Moses, saying, "What shall we drink?"  25 Then he cried to Yahweh. Yahweh shown 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;