Ill go one better - a coin whose legend is the words Constantine saw in the vision.
HOC SIGNO VICTOR ERIS - 'By this sign you shall Conquer' (it actually says 'This sign victor you will be' which id interpret as 'this sign will make you the victor'):
Issued by Vetranio for Constantius II (Vetranio was a top notch guy who usurped power in order to protect Rome from Magnentius whilst constantius was in Persia - after his victory against magnentius, he waited. When his army met Constantius' army, he went to the emperor and relinquished power immediately)
This coin has been looked at by the Peeps at the British museum, no less, who could not differentiate it between Vetranio and Constantius. I maintain it to be Constantius, which is the more common of the two:


I also have some cut up Magnentius folles with the Chi-rho on them, but the pictures dont come out so well. Ones been Damnatio'd with a punch through the eye.