The “touch-stone of virtue,” given to Alasnam by one of the Genii. If he looked in this mirror it informed him whether a damsel would remain to him faithful or not. If the mirror remained unsullied so would the maiden; if it clouded, the maiden would prove faithless. —Arabian Nights: Prince Zeyn Alasnam.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894