We also saw there at that time [Dec 1145] the aforesaid Bishop of Jabala in Syria.... He said, indeed, that not many years since, one John, a king and priest living in the Far East, beyond Persia and Armenia, and who, with his people, is a Christian, but a Nestorian, had warred upon the so­called Samiards, the brother kings of the Medes and Persians. John also attacked Ebactanus . I . the capital of their kingdom. When the aforesaid kings advanced against him with a force of Persians, Medes, and Assyrians, a three­day struggle ensued, since both sides were willing to die rather than to flee. At length, Prester John ­ so he is usually called ­ put the Persians to flight and emerged from the dreadful slaughter as victor. The Bishop said that the aforesaid John moved his army to aid the church of Jerusalem, but that when he came to the Tigris and was unable to take his army across it by any means, be turned aside to the north, where he had been informed that the stream was frozen solid during the winter. There he awaited the ice for several years, but saw none because of the temperate weather. His army lost many men on account of the weather to which they were unaccustomed and he was compelled to return home. He is said to be a descendant of the Magi of old, who are mentioned in the Gospel.31 He governs the same people as they did and is said to enjoy such glory and such plenty that be uses no scepter save one of emerald. Fired by the example of his forefathers, who came to adore Christ in the manger, he proposed to go to Jerusalem, but he was, they say, turned back for the aforementioned reason.

 Otto of Freising, Chronicon, ed. G.H. Pertz, MGH SSRG (Hanover: Hahn, 1867), VII, 33, (pp. 334-35), translated by James Brundage, The Crusades: A Documentary History, (Milwaukee, WI: Marquette University Press, 1962)