Plato, Laws 723c

καλῶς μὲν τοίνυν, ὦ Κλεινία, δοκεῖς μοι τό γε τοσοῦτον λέγειν, ὅτι πᾶσίν γε νόμοις ἔστιν προοίμια καὶ ὅτι πάσης ἀρχόμενον νομοθεσίας χρὴ προτιθέναι παντὸς τοῦ λόγου τὸ πεφυκὸς προοίμιον ἑκάστοις—οὐ γὰρ σμικρὸν τὸ μετὰ τοῦτό ἐστιν ῥηθησόμενον, οὐδʼ ὀλίγον διαφέρον ἢ σαφῶς ἢ μὴ σαφῶς αὐτὰ μνημονεύεσθαι—τὸ μέντοι μεγάλων πέρι λεγομένων νόμων καὶ σμικρῶν εἰ ὁμοίως προοιμιάζεσθαι προστάττοιμεν, οὐκ ἂν ὀρθῶς λέγοιμεν. οὐδὲ γὰρ ᾄσματος

You are right, I believe, Clinias, in asserting at least thus much,—that all laws have preludes, and that, in commencing each piece of legislation, one ought to preface each enactment with the prelude that naturally belongs to it—for the statement that is to follow the prelude is one of no small importance, and it makes a vast difference whether these statements are distinctly or indistinctly remembered; still, we should be wrong if we prescribed that all statutes, great and small, should be equally provided with preludes.