Right, well, I've read the quote out of the Historia, it is available on google books, or at least, it was when I read several months ago.
The Loeb edition of the Augustan History is online, thanks to Bill Thayer. The Lives of Firmus, Saturninus, Proculus and Bonosus
is the bit we want. These were a bunch of minor usurpers, and the lives are nearly entirely fiction, or so I understand.
The bit we're interested in is the life of Saturninus, a contemporary of Aurelian:
7 Saturninus17 was a Gaul by birth, one of a nation that is ever most restless and always desirous of creating either an emperor or an empire.18
2 To this man, above all the other generals, because it seemed certain that he was truly the greatest, Aurelian had p399given the command of the Eastern frontier, wisely charging him never to visit Egypt.19
3 For, as we see, this far-sighted man was well acquainted with the Gallic character and feared that if Saturninus visited this turbulent land he might be drawn by association with the inhabitants to a course toward which he was by nature inclined.
4 For the Egyptians, as you know well enough, are puffed up, madmen,20 boastful, doers of injury, and, in fact, liars and without restraint, always craving something new, even in their popular songs, writers of verse, makers of epigrams, astrologers, soothsayers, quacksalvers.
5 Among them, indeed, are Christians and Samaritans and those who are always ill-pleased by the present, though enjoying unbounded liberty.
6 But, lest any Egyptian be angry with me, thinking that what I have set forth in writing is solely my own, I will cite one of Hadrian's letters, taken from the works of his freedman Phlegon,21 which fully reveals the character of the Egyptians.
8 From Hadrian Augustus to Servianus22 the consul, greeting. The land of Egypt, the praises of which you have been recounting to me, my dear Servianus, I have found to be wholly light-minded, unstable, and blown about by every breath of rumour. 2 There those who worship Serapis are, in fact, Christians, and those who call themselves bishops of Christ are, in fact, devotees of Serapis. 3 There is no chief of the Jewish synagogue, no Samaritan, no Christian presbyter, who is not an astrologer, a soothsayer, or an anointer. 4 Even the Patriarch himself, when he comes to Egypt, is forced by some to worship Serapis, p401by others to worship Christ. 5 They are a folk most seditious, most deceitful, most given to injury; but their city is prosperous, rich, and fruitful, and in it no one is idle. 6 Some are blowers of glass, others makers of paper, all are at least weavers of linen23 or seem to belong to one craft or another; the lame have their occupations, the eunuchs have theirs, the blind have theirs, and not even those whose hands are crippled are idle. 7 Their only god is money, and this the Christians, the Jews, and, in fact, all nations adore. And would that this city had a better character, for indeed it is worthy by reason of its richness and by reason of its size to hold the chief place in the whole of Egypt. 8 I granted it every favour, I restored to it all its ancient rights and bestowed on it new ones besides, so that the people gave thanks to me while I was present among them. Then, no sooner had I departed thence than they said many things against my son Verus,24 and what they said about Antinous25 I believe you have learned. 9 I can only wish for them that they may live on their own chickens, which they breed in a fashion I am ashamed to describe.26 10 I am sending you over some cups, changing colour27 and variegated, presented to me by the priest of a temple and now dedicated particularly to you and my sister. I should like you to use them at banquets on feast-days. Take good care, however, that our dear Africanus28 does not use them too freely."
9 So then, holding such an opinion about the p403Egyptians Aurelian forbade Saturninus to visit Egypt, showing a wisdom that was truly divine. ...
Interesting that the material is supposedly from Phlegon. I don't know whether the letter is considered genuine or not. I don't see why it should not be. The works of Phlegon were extant in the fourth century, I believe, since Eusebius and St. Jerome use them.
However, I was not yet aware that it is considered a 4th century forgery. Well, that's a bit of a shame.
The work purports to be by six historians. But once you analyse who did what, who wrote what, etc, it apparently starts to get confused, and you come to the conclusion that all these six are in fact one man. The purpose of the composition is unknown. Some have speculated that it has an anti-Christian purpose to it.
So since the objective of the rumor appears to be for the defamation of these particular Egyptian Christians, do you think there is any truth to the rumor at all, in spite of it not having originated with Hadrian?
All the best,