= Timaeus, Platonic Lexicon 986a.20; cf. a similar entry at 988a.20, which is also adopted by Photius, Lexicon epsilon1427 Theodoridis.
Much less similar (though cited by Adler as a comparandum) is Gregory of Corinth, De dialecta Attica 58. Adler notes that Naber rashly attributed this to Aelius Dionysius, but does not seem to have discovered the true source herself. At issue is the first-person ending of the pluperfect active forms of the verbs in question. Timaeus is explaining that Plato uses forms ending in -h (e.g. Symposium 198C, Republic 1.329B), whereas one would expect a classical Athenian author to use the Attic alternative -ein (at least one would if one were brought up, as Timaeus probably was, on a steady diet of Demosthenes, who uses this ending more than any other author).
[1] A past stative verb, pluperfect in form; hence the translation with the English simple past.