From LSJ

Εἰ θνητὸς εἶ, βέλτιστε, θνητὰ καὶ φρόνει → Mortalis quum sis, intra mortalem sape → Bist sterblich du, mein Bester, denk auch Sterbliches

Menander, Monostichoi, 173

Frisk Etymological English

Grammatical information: preterite prefix
Meaning: the augment (Il.).
Other forms: rarely ἠ- (s. below)
Dialectal forms: rare in Mycenaean, apedoke /ap-e-doke/.
Origin: IE [Indo-European] *h₁e- augment
Etymology: Old element, also found in Indo-Iiranian (a-, ā-), Armenian (e-) and Phrygian (e-), e. g. ἔ-φερε = Skt. á-bharat, Arm. e-ber; Phryg. ἔ-δαες ἔθηκε. See Schwyzer 651ff.; on the form ἠ- s. Debrunner, Festschrift Zucker 85ff; Rix, Hist. Gramm. d. griech. 226ff. Often long vowel through contractieon: *e-h₂ege-t > ἠ̃γε. A long augment analogically from ἠθελον < *e-h₁dhel-, e.g. in ήβουλόμην. Probably all long augments are analogical (Ruijgh, Lingua 28, 1971, 166.