The spear of the the angel Ithuriel, the slightest touch of which exposed deceit. Hence, when Satan squatted like a toad “close to the ear of Eve,” Ithuriel only touched the creature with his spear, and it resumed the form of Satan.
Him [i.e. Satan], thus intent Ithuriel with his spear Touched lightly; for no falsehood can endure Touch of celestial temper, but returns Of force to its own likeness.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894