Laudator Temporis Acti

David E. Aune, Revelation 1-5 (Dallas: Word Books, 1997), p. 123, on Rev. 2.7 (ὁ ἔχων οὖς ἀκουσάτω τί τὸ πνεῦμα λέγει ταῖς ἐκκλησίαις = He that hath an ear, let him hear what the Spirit saith unto the churches):

Placed at the conclusion of each of the seven proclamations, this formula functions as a proclamation formula, i.e., as an injunction to an audience to pay very close attention to the message that it accompanies. Dibelius coined the term Weckformel, "alertness formula," for the parallels found in the synoptic gospels (Die Formgeschichte des Evangeliums, 6th ed., ed. G. Bornkamm [Tübingen: Mohr-Siebeck, 1971] 248). This formula has no close verbal parallels in ancient literature with the exception of the parable tradition found in the synoptic Gospels and in some apocryphal gospels.

There are a number of parallels from ancient pagan literature, although they lack the characteristic third person imperative of the Weckformel. See, e.g., Heraclitus fragment 34 Diels (tr. Kathleen Freeman):

Not understanding, although they have heard, they are like the deaf.

ἀξύνετοι ἀκούσαντες κωφοῖσιν ἐοίκασι.

and Aeschylus, Prometheus Bound 447-448 (tr. Herbert Weir Smyth):

First of all, though they had eyes to see, they saw to no avail; they had ears, but understood not.

οἳ πρῶτα μὲν βλέποντες ἔβλεπον μάτην,
κλύοντες οὐκ ἤκουον.

According to [Demosthenes] 25.89 (tr. J.H. Vince), the expression was proverbial:

As the saying runs, "seeing, they see not; hearing, do not hear."

τὸ τῆς παροιμίας, ὁρῶντας μὴ ὁρᾶν καὶ ἀκούοντας μὴ ἀκούειν.

For more, see here.