Sh'mot (Exo) 15:23-26

They arrived at Marah but couldn’t drink the water there, because it was bitter. This is why they called it Marah [bitterness]. The people grumbled against Moshe and asked, “What are we to drink?” Moshe cried to ADONAI; and ADONAI showed him a certain piece of wood, which, when he threw it into the water, made the water taste good. There ADONAI made laws and rules of life for them, and there he tested them. He said, “If you will listen intently to the voice of ADONAI your God, do what he considers right, pay attention to his mitzvot and observe his laws, I will not afflict you with any of the diseases I brought on the Egyptians; because I am ADONAI your healer.” (A: v, S: iv)