25 Then Moses cried out to the LORD, and the LORD showed him a piece of wood. He threw it into the water, and the water became fit to drink. There the LORD issued a ruling and instruction for them and put them to the test.

Read Exodus 15:25 Using Other Translations

And he cried unto the LORD; and the LORD 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,
And he cried to the LORD, and the LORD showed him a log, and he threw it into the water, and the water became sweet. There the LORD made for them a statute and a rule, and there he tested them,
So Moses cried out to the LORD for help, and the LORD showed him a piece of wood. Moses threw it into the water, and this made the water good to drink. It was there at Marah that the LORD set before them the following decree as a standard to test their faithfulness to him.