Polybius 5.104.1 (reporting a speech by Agelaus of Naupactus; tr. Ian Scott-Kilvert):
It would be best if the Greeks never went to war with one
another, if they could regard it as the greatest gift of the gods for
them all to speak with one voice, and could join hands like men
who are crossing a river; in this way they could unite to repulse the
incursions of the barbarians and to preserve themselves and their
ὃς ἔφη δεῖν μάλιστα μὲν μηδέποτε πολεμεῖν τοὺς Ἕλληνας ἀλλήλοις, ἀλλὰ μεγάλην χάριν ἔχειν τοῖς θεοῖς, εἰ λέγοντες ἓν καὶ ταὐτὸ πάντες καὶ συμπλέκοντες τὰς χεῖρας, καθάπερ οἱ τοὺς ποταμοὺς διαβαίνοντες, δύναιντο τὰς τῶν βαρβάρων ἐφόδους ἀποτριβόμενοι συσσῴζειν σφᾶς αὐτοὺς καὶ τὰς πόλεις.