Polybius 12.25a.1-2 (tr. Ian Scott-Kilvert):
There is a proverb which tells us that a single drop, taken from
even the largest vessel, is enough to reveal to us the nature of the
whole contents, and the same principle may be applied to the
subject we are now discussing. Accordingly when we find one or
two false statements in a book and they prove to have been
deliberately made, we know that we can no longer treat anything
that is said by such an author as reliable or trustworthy.
καθάπερ γὰρ ἐκ τῶν παροιμιῶν ἱκανὸν εἶναί φασι σταλαγμὸν ἕνα τοῦ μεγίστου τεύχους εἰς τὸ γνῶναι τὸ πᾶν ἔγχυμα, τὸν αὐτὸν τρόπον καὶ περὶ τῶν ὑποκειμένων χρὴ διαλαμβάνειν· ἐπειδὰν γὰρ ἓν ἢ δεύτερον εὑρεθῇ ψεῦδος ἐν τοῖς συγγράμμασι, καὶ τοῦτο γεγονὸς ᾖ κατὰ προαίρεσιν, δῆλον ὡς οὐδὲν ἂν ἔτι βέβαιον οὐδ᾽ ἀσφαλὲς γένοιτο τῶν ὑπὸ τοῦ τοιούτου συγγραφέως λεγομένων.