“Choose your friends, then treat them as friends; do not regard them like slaves or servants, but associate with them frankly and simply and generously; not saying one thing of them and thinking something else.”

Myth at the end of Julian's oration to the cynic Heracleios, as translated in The Emperor Julian : Paganism and Christianity (1879) http://www.third-millennium-library.com/MedievalHistory/Julian_the_Emperor/CHAPTER_VI.html by Gerald Henry Rendall, Ch. VI : Julian's Personal Religion, p. 138
General sources
Context: "Suppose that I and Athene, at the behest of Zeus", said Helios, "were to make you steward of all these in the room of him that hath the inheritance." Then the young man clung to him once more, and besought him greatly that he might remain there. But he said, "Be not very rebellious, lest the excess of my love be turned to the fierceness of hatred."
So the young man answered, "Most mighty Helios, and thee Athene, and Zeus himself, I do adjure, do with me what ye will."
After this Hermes, suddenly reappearing, filled him with new courage, for now he thought he had found a guide for his return journey, and his sojourn on earth. And Athene said, "Listen, most goodly child of mine and of this good sire divine! This heir, you see, finds no pleasure in the best of his shepherds, while the flatterers and rogues have made him their subject and slave. Consequently the good love him not, while his supposed friends wrong and injure him most fatally. Take heed therefore when you return, not to put the flatterer before the friend. Give ear, my son, to yet a second admonition. Yon sleeper is habitually deceived; do you therefore be sober and watch, that the flatterer may never deceive and cheat you by a show of friendly candor, just as some sooty and grimy smith by dressing in white and plastering his cheeks with enamel might finally induce you to give him one of your daughters to wife. List now to a third admonition. Set a strong watch upon yourself: reverence us and us alone, and of men him that is like us and none other. You see what tricks self-consciousness and dumb-foundering faint-heartedness have played with yonder idiot." Great Helios here took up the discourse and said, "Choose your friends, then treat them as friends; do not regard them like slaves or servants, but associate with them frankly and simply and generously; not saying one thing of them and thinking something else. See how distrust towards friends has damaged yonder heritor. Love your subjects as we love you. Let respect toward us take precedence of all goods: for we are your benefactors and friends and saviours."
At these words the young man's heart was full, and he made ready there and then to obey the Gods implicitly always. "Away, then", said Helios, "and good hope go with you. For we shall be with you everywhere, I and Athene and Hermes here, and with us all the Gods that are in Olympus, and Gods of the air and of the earth, and all manner of deities everywhere, so long as you are holy toward us, loyal to your friends, kindly to your subjects, ruling and guiding them for their good. Never yield yourself a slave to your own desires or theirs. …"