An article by Dominic White OP, “’My mystery is for me’: a saying of Jesus?” Scripture Bulletin 44 (2014), 27-42, draws attention to the appearance of an agraphon in Testamentum Domini 18: “For my mysteries are given to those who are mine”. This agraphon appears elsewhere, notably in Clement Strom. 5.10, ps-Clement Rec. 19.19, and in Chrysostom Hom. in I Cor. on I Cor. 2:6-7.
It is also attributed to Isaiah by Theodoret of Cyrus In Psalmos on Ps 24 (25):14 (PG80.1041), where the text reads τὰ μυστήρια μου ἐμοὶ καὶ τοῖς ἐμοῖς, and again with reference to Ps 65 (66):16 (PG80.1369), though here it is not attributed. It is also attributed to Isaiah (probably) by Jerome Ep. 48.13 (he attributes it to “the prophet”): “My mystery is for me,” says the prophet; “my mystery is for me and for them that are mine.” This reference is unobserved by White.
Isaiah 24:16b in the Vulgate reads: “Et dixi: Secretum meum mihi, secretum meum mihi.” Jerome further notes at Comm. in Is. 8.24 that the reading is attested by Theodotion. This reading is attested in one LXX stream with the reading: “και ειπεν το μυστηριον μου εμοι το μυστηριον μου εμοι”. One MS extends this with “και τοις εμοις.” The Hebrew text rendered is רזי לי, so it would seem that this MS has extended the text by citing the agraphon as received, rather than as found in the text. Even so it is noteworthy that when Jerome cites the passage he does so in a fuller form than that found in his version of Isaiah, again indicating that he had independent knowledge of the statement.
It is thus hard to say whether the agraphon was received by the redactor of Testamentum Domini (this passage certainly being redactional) as from Jesus directly or as from him through Isaiah, though given that it is in a fuller form, it seems more probable that it was received as from the direct testimony of Jesus. What is noteworthy is that here, as in the second citation from Theodoret, the saying is joined to Matt. 7:6. The clear intention of the redactor is to limit the instructions here received from Jesus to a select group, the ascetics of the Testamentum community.