exodus 15:25 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, - Free Bible Online

      25. the Lord showed him a tree, which when he had cast into the waters, the waters were made sweet--Some travellers have pronounced this to be the Elvah of the Arabs--a shrub in form and flower resembling our hawthorn; others, the berries of the Ghurkhud--a bush found growing around all brackish fountains. But neither of these shrubs are known by the natives to possess such natural virtues. It is far more likely that God miraculously endowed some tree with the property of purifying the bitter water--a tree employed as the medium, but the sweetening was not dependent upon the nature or quality of the tree, but the power of God (compare Joh 9:6). And hence the "statute and ordinance" that followed, which would have been singularly inopportune if no miracle had been wrought.
      and there he proved them--God now brought the Israelites into circumstances which would put their faith and obedience to the test (compare Ge 22:1).

JFB.