Procopius, History of the Wars
1.5.7-8 (tr. H.B. Dewing):
But they were altogether unwilling to put to death a
man of the royal blood,
and decided to confine him in a castle which it is their habit to call the "Prison of Oblivion." For if anyone is cast into it,
the law permits no mention of him to be made thereafter,
but death is the penalty for the man who speaks his name; for this reason it has received this title among the Persians.
οἱ δὲ κτεῖναι ἄνδρα τοῦ βασιλείου αἵματος οὐδ᾿ ὅλως ἔγνωσαν,
ἀλλ᾿ ἐν φρουρίῳ καθεῖρξαι ὅπερ τῆς Λήθης καλεῖν 8νενομίκασιν. ἢν γάρ τις ἐνταῦθα ἐμβληθεὶς τύχῃ, οὐκέτι ὁ νόμος ἐφίησι μνήμην αὐτοῦ εἶναι, ἀλλὰ θάνατος τῷ ὠνομακότι ἡ ζημία ἐστί· διὸ δὴ καὶ τὴν ἐπωνυμίαν ταύτην πρὸς Περσῶν ἔλαχεν.