Enchiridion · §37

ὑπὲρ δύναμιν εἰ πρόσωπόν τι ἔλαβες, ἠσχημόνησας μὲν ἐν τούτῳ, παρῆκας δὲ ὃ ἠδύνασο πληρῶσαι.

huper dunamin ei prosōpon ti elabes, ēschēmonēsas men en toutō, parēkas de ho ēdunaso plērōsai.

If you take on a role that is beyond your capacity, you both disgrace yourself in that role and neglect the role you were actually capable of filling well.

— Enchiridion 37

Source: Greek (Schenkl/Oldfather critical edition tradition) + Long 1877 English translation