I always fear less a dull man who is naturally strong
Than someone who is weak and clever.
 
[ἀεὶ γὰρ ἄνδρα σκαιὸν ἰσχυρὸν φύσει
ἧσσον δέδοικα τἀσθενοῦς τε καὶ σοφοῦ.]

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

Bellerophon [Βελλεροφῶν], frag. 290 (TGF) (c. 430 BC) [tr. @sentantiq (2015)]
    (Source)

Barnes frag. 51, Musgrave frag. 11. (Source (Greek)). Alternate translations:

By far less dangerous I esteem the fool
Endued with strength of body, than the man
Who's feeble and yet wise.
[tr. Wodhull (1809)]

I always fear a stupid if bodily powerful man less than one who is both weak and clever.
[tr. Collard, Hargreaves, Cropp (1995)]

Always I fear an unintelligent but naturally strong man less than a weak and clever one.
[tr. Stevens (2012)]

I fear less the powerful but stupid
than the weak and cunning.
[Source]