A pious Brahmin, it is written, made a vow that on a certain day he would sacrifice a sheep, and on the appointed morning he went forth to buy one. There lived in his neighbourhood three rogues who knew of his vow, and laid a scheme for profiting by it. The first met him and said, “Oh Brahmin, wilt thou buy a sheep? I have one fit for sacrifice.”
“It is for that very purpose,” said the holy man, “that I came forth this day.” Then the impostor opened a bag, and brought out of it an unclean beast, an ugly dog, lame and blind. Thereon the Brahmin cried out, “Wretch, who touchest things impure, and utterest things untrue, callest thou that cur a sheep?” “Truly,” answered the other, “it is a sheep of the finest fleece, and of the sweetest flesh. Oh Brahmin, it will be an offering most acceptable to the gods.”
“Friend,” said the Brahmin, “either thou or I must be blind.”
Just then one of the accomplice’s came up. “Praised be the gods,” said this second rogue, “that I have been saved the trouble of going to the market for a sheep! This is such a sheep as I wanted. For how much wilt thou sell it?” When the Brahmin heard this, his mind waved to and fro, like one swinging in the air at a holy festival. “Sir,” said he to the new comer, “take heed what thou dost; this is no sheep, but an unclean cur.”
“Oh Brahmin,” said the new comer, “thou art drunk or mad!”
At this time the third confederate drew near. “Let us ask this man,” said the Brahmin, “what the creature is, and I will stand by what he shall say.” To this the others agreed; and the Brahmin called out, “Oh stranger, what dost thou call this beast?”
“Surely, oh Brahmin,” said the knave, “it is a fine sheep.” Then the Brahmin said, “Surely the gods have taken away my senses;” and he asked pardon of him who carried the dog, and bought it for a measure of rice and a pot of ghee, and offered it up to the gods, who, being wroth at this unclean sacrifice, smote him with a sore disease in all his joints.
Thomas Babington Macauley, "Mr. Robert Montgomery (I)" (Edinburgh Review, April 1830), Critical, Historical, and Miscellaneous Essays, Volume 2. Macauley attributes the story to Pilpay, i.e., the Panchatantra. In the version translated later by Ryder, the story works in reverse: the priest has a nice clean animal and the rogues con him out of it by convincing him that it is unclean; but there are many different versions of the work.