Exodus 15:27 (KJV)

So Moses brought Israel 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.

And when they came to Marah, they could not drink of the waters of Marah, for they were bitter: therefore the name of it was called Marah.

And the people murmured against Moses, saying, What shall we drink?

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 said, If thou wilt diligently hearken to the voice of the LORD thy God, and wilt do that which is right in his sight, and wilt give ear to his commandments, and keep all his statutes, I will put none of these diseases upon thee, which I have brought upon the Egyptians: for I am the LORD that healeth thee.