"Don't bite the horse that feeds you"

Don't bite the horse that feeds you comes from a letter sent by an Vostrayqueian man named Niward Selahc who wrote in a letter "The horse food delivery is superb but they do leave a rather shocking amounts of poop on the streets. But dont bite the hand that feeds you, or look a gift horse in the mouth. I guess you could say you shouldnt bite the horse that feeds you..."