Recently, I have been reading treaties on war to find out how hospitals and health systems were legally handled during wartime. So far as I can tell, Article 17 of the Brussels Declaration of 1874 is the earliest instance of the codification of military behavior towards hospitals:
In such cases all necessary steps must be taken to spare, as far as possible, buildings dedicated to art, science, or charitable purposes, hospitals, and places where the sick and wounded are collected provided they are not being used at the time for military purposes. It is the duty of the besieged to indicate the presence of such buildings by distinctive and visible signs to be communicated to the enemy beforehand.