E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
When flatterers meet, the devil goes to dinner. Flattery is so pernicious, so fills the heart with pride and conceit, so perverts the judgment and disturbs the balance of the mind, that Satan himself could do no greater mischief. He may go to dinner and leave the leaven of wickedness to operate its own mischief.
Porteus, there is a proverb thou shouldst read;
When flatterers meet, the devil goes to dinner.