The Bronze Serpent
Serpents in the camp, and one of bronze lifted on a pole: whoever looked, lived. Scroll to watch it.
Make thee a serpent and set it for a sign and it shall be, if a serpent have bitten a man, that every one that is bitten, seeing it shall live.
Numbers 21:8, as St Cyril of Alexandria gives it
This was the figure which Moses completed by fixing the serpent to a cross, that whoso had been bitten by the living serpent, and looked to the brasen serpent, might be saved by believing. Does then the brazen serpent save when crucified, and shall not the Son of God incarnate save when crucified also? On each occasion life comes by means of wood.
Cyril of Jerusalem, Catechetical Lectures XIII.20