Shemot 15:23-25

And when they came to Marah (Bitter Place), they could not drink of the mayim from Marah, for they were marim; therefore the shem of it was called Marah. So the people murmured against Moshe, saying, Mah nishteh (what shall we drink)? And he cried unto HASHEM; and HASHEM showed him an etz (tree) which when he had cast into the mayim, the waters were made sweet; at that place He made for them a chok (statute, requirement, obligation) and a mishpat (divine judgment), and there He tested them