...equo ne credite, Teucri. Quidquid id est, timeo Danaos et dona ferentes."
- Vergil, Aeneid, II.48-49
Literally, "Do not trust the horse, Trojans. Whatever it is, I fear Greeks, even those bearing gifts." This is the original source of the proverb - the Roman poet Vergil's tale of the founding of Rome by the hero Aeneas, leader of the last of the Trojans.
Words to live by - one had better not trust a gift given by someone they have reason not to trust. Of course, it's worth noting that the character who utters this line - the Trojan priest Laocoon - and his two sons are devoured shortly thereafter by a pair of serpents sent by Poseidon, who was sympathetic to the Greek cause. If the gift-giver has powerful friends, it's probably best that you keep your distrust private, unless martyrdom appeals to you.