Plato, Epinomis 979e

οὐ γὰρ σμικρά, ὦ Κλεινία· τὸ δὲ χαλεπώτερον, ὅτι παντάπασι καὶ πάντως ἀληθῆ. σφόδρα γε, ὦ ξένε· ἀλλʼ ὅμως μὴ ἀποκάμῃς λέγων ὃ φῄς. ναί, μηδὲ σφὼ τοίνυν ἀκούοντε. ταῦτʼ ἔσται· καὶ ὑπὲρ ἀμφοῖν ἐγώ σοι φράζω.

Yes, for they are not small, Cleinias: but what is more difficult is to show that they are entirely and in every sense true. Very much so, good sir: but still, do not weary of the task of stating your views. I will not, and therefore you two must not weary either of listening to me. Agreed: I give you my word for us both.