I think you’re thinking of Liutprand of Cremona, not-too-diplomatic diplomat to Constantinople, who spent a fair while under lock and key:
His reception at Constantinople was humiliating and ultimately futile after the subject of Otto’s claim to the title [Holy Roman] Emperor caused friction, triggered by a letter from Pope John XIII which offensively addressed Nicephorus as “the emperor of the Greeks”.
(Otto I had just launched or revived the Holy Roman Empire, depending on whether Charlemagne counts as the same empire.)
On his second mission to Constantinople, for instance, after his purple purchases are confiscated, he tells the imperial party that at home whores and conjurers wear purple.
As other answers have said, the Byzantines never ceased considering themselves the true Roman Empire, and they referred to themselves as Romans, not Greeks; they were careful to use the term rēx ‘king’ and not basileus ‘emperor’ for the rulers of the West.