Isocrates 1.41 (tr. George Norlin):
Always when you are about to say anything, first weigh it in your mind; for with many the tongue outruns the thought. Let there be but two occasions for speech—when the subject is one which you thoroughly know and when it is one on which you are compelled to speak. On these occasions alone is speech better than silence; on all others, it is better to be silent than to speak.
πᾶν ὅ τι ἂν μέλλῃς ἐρεῖν, πρότερον ἐπισκόπει τῇ γνώμῃ· πολλοῖς γὰρ ἡ γλῶττα προτρέχει τῆς διανοίας. δύο ποιοῦ καιροὺς τοῦ λέγειν, ἢ περὶ ὧν οἶσθα σαφῶς, ἢ περὶ ὧν ἀναγκαῖον εἰπεῖν. ἐν τούτοις γὰρ μόνοις ὁ λόγος τῆς σιγῆς κρείττων, ἐν δὲ τοῖς ἄλλοις ἄμεινον σιγᾶν ἢ λέγειν.