“Eustace, which first was named Placidus, was master of the chivalry of Trajan, the emperor, and was right busy in the works of mercy, but he was a worshipper of idols. And he had a wife of the same rite, and also of the deeds of mercy, of whom he had two sons, which he did do nourish after his estate. And because he was ententive to the works of mercy, he deserved to be enlumined to the way of truth.

So on a day, as he was on hunting, he found an herd of harts, among whom he saw one more fair and greater than the other, which departed from the company and sprang into the thickest of the forest. And the other knights ran after the other harts, but Placidus siewed him with all his might, and enforced to take him. And when the hart saw that he followed with all his power, at the last he went up on a high rock, and Placidus approaching nigh, thought in his mind how he might take him. And as he beheld and considered the hart diligently, he saw between his horns the form of the holy cross shining more clear than the sun, and the image of Christ, which by the mouth of the hart, like as sometime Balaam by the ass, spake to him, saying: Placidus, wherefore followest me hither? I am appeared to thee in this beast for the grace of thee. I am Jesu Christ, whom thou honourest ignorantly, thy alms be ascended up tofore me, and therefore I come hither so that by this hart that thou huntest I may hunt thee”

Source:

‘The Golden Legend or Lives of the Saints’. Compiled by Jacobus de Voragine, Archbishop of Genoa, 1275. First Edition Published 1470. Englished by William Caxton, First Edition 1483, Edited by F.S. Ellis, Temple Classics, 1900 (Reprinted 1922, 1931.)

http://legacy.fordham.edu/halsall/basis/goldenlegend/GoldenLegend-Volume6.asp#Eustace