The Denkard, a summary of the partly lost Great Avesta of the sixth century, contains the story Zarathustra and his protector Hystaspes. Denkard 7.4.65-87 is offered here in the translation by Mary Boyce.
Zarathustra and Hystaspes
 Zarathustra departed alone, at the counsel and command of Ahuramazda, to the court of Hystaspes and the ordeal of that terrible contest [between him and his opponents]. [...]
 Hystaspes, through his great wisdom, rightness of thought and spiritual beliefs, would have been ready to listen to Zarathustra's words in order to judge of his prophethood; but even before he could hear Zarathustra's words and recognize his nature, Hystaspes was turned against Zarathustra, through slander and sorcery, by the persuasions of the wicked rulers and the priests of the old religion. Then he consigned Zarathustra to imprisonment and torture. [...]
 And lo, through the strength of Zarathustra, who, alone fought a terrible battle against evil, a great miracle was manifest to king Hystaspes and his retainers, when they found him alive and full of glory, despite hardship and fetters and other afflictions and prolonged starvation.
[Zarathustra then works miracles from his prison, among them a cure of Hystaspes' favorite war horse.]
 Further, through his spiritual insight he was able to tell and make known the thoughts of king Hystaspes and his fellow countrymen, and many other hidden things. [...]
 Then in order to vindicate the faith, and to reveal its truth and wisdom, and to remove all doubts from king Hystaspes, [...] the creator Ahuramazda sent [the spirit] Good Thought and [the spirit] Truth and Holy Fire as messengers to Hystaspes, to make known the truth of Zarathustra's prophethood, and the will of Ahuramazda that Hystaspes should accept the Mazda-worshipping Religion and make it current in the world. The miracle was revealed to the people of the land of Hystaspes when the Holy Immortals descended form sky to earth and entered the dwelling of Hystaspes. [...]
 The Fire of Ahuramazda said in a man's voice: "Fear not, illustrious king Hystaspes, for you have no cause for dread. [...] This is the message we bring you: Accept then, most justly and wisely, the vision which Spitama Zarathustra has expounded in purity. Chant the Holiest of All Prayers, praise perfect righteousness and deny all worship to the Demons. For Ahuramazda desires that you should adhere to this Religion, and the Holy Immortals desire it."