The following comes from Catholic Pages:
A popular story springs up about St. Anthony, mention of it is made in the official Butler's lives of the Saints. It is the story of St. Anthony's encounter with an Jewish man. This man contested the "Real Presence of the Eucharist; so one day he approached St. Anthony publicly and after spelling out all of his reasons for not believing in such a "fable" as the Real Presence, he challenged Anthony to a contest. He bet that the Real Presence was a lie, and he proposed to "starve a donkey" for three days---no hay, and see if the donkey would choose to eat hay or preferred the Eucharist. St. Anthony, being publicly "put on the spot" accepted the challenge. So the wealthy merchant brought out his donkey, publicly hitched him to a post where he could be observed by all; and proceeding to "starve the donkey" for three days. Simultaneously, St. Anthony went into the forest and "fasted" for three days---taking no food. When the day of trial came, Anthony emerged from the forest and sought out a local Church where he took the Eucharist and returned to the spot where the donkey was tied. Meanwhile, the challenger had placed a large pile of hay about 20 ft away from the donkey. Anthony took out the Eucharist holding it in his hands; the merchant untied the donkey, who needless to say, made a "beeline" for the pile of hay. Just as the donkey was about to reach the hay, St. Anthony elevated the Eucharist and shouted in a loud voice: "Mule, in the Name of the Lord Our God, I command you to come here and adore your Creator ! " The donkey "reared up" on his hind legs as if someone had pulled him by a bridle; he spun around, and ran to St. Anthony, dropping to his forelegs--- hind legs still extended; and put his head down to the ground---in a "posture of adoration" before the Eucharist which St. Anthony continued to hold elevated. The Jewish merchant, stunned by what occurred begged St. Anthony's forgiveness, converted on the spot, and donated the money to build a new Catholic Church, his newfound faith. On the cornerstone of the Church, he had engraved a picture of St. Anthony holding the Eucharist aloft and the donkey, "kneeling on his forepaws" in adoration of the Body of Christ.
It's funny that nowadays, people find such things difficult to believe. We have become so sophisticated, so urban, so cosmopolitan. Yet, St. Louis de Montfort once said that when you hear these miraculous stories about the saints; believe them. The worldly find them difficult to believe, but to the faithful, it is not surprising that God does great things for those who truly follow Him.