If the gods do a shameful thing, they are not gods.

[εἰ θεοί τι δρῶσιν αἰσχρόν, οὐκ εἰσὶν θεοί.]

Euripides (485?-406? BC) Greek tragic dramatist

Bellerophon [Βελλεροφῶν], frag. 292, l. 7 (TGF) (c. 430 BC) [tr. @sentantiq (2014)]
    (Source)

Barnes frag. 112, Musgrave frag. 19. (Source (Greek)). Alternate translations:

But to thee
This I maintain, that if the Gods commit
Aught that is base, they are no longer Gods.
[tr. Wodhull (1809)]

If gods do anything shameful, they are not gods.
[tr. Collard, Hargreaves, Cropp (1995)]

If gods do what is shameful, they are not gods.
[tr. Stevens (2012), frag. 286b]

If the gods do anything base, they are not gods.
[tr. Dixon (2014)]