Exodus 15:23-25:

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,

See also:

Healing and Atonement
How Could Jesus Heal the Sick Based on His Future Atonement?