I have gotten lately into late antiquity history and through some books i have come across this particular quote about the sassanian palace in the capital:

" Inside the audience hall there were said to be three thrones kept permanently ready for the other three great rulers of the world - the emperor of Rome, the emperor of China, and the Khagan of the Turks - ready, that is, for when they should come to submit to the Sasanian emperor"

so i want to ask if anyone knows the original source for it, thanks.