Exodus 19:15: And he said to the people, ‘Prepare for the third day; do not go near a woman'.

Clement of Alexandria, Stromateis 3.73
ὅθεν εἰκότως καὶ ὁ Μωυσῆς κατ’ ὀλίγον εἰς ἐγκράτειαν προβιβάζων τοὺς Ἰουδαίους " τριῶν ἡμερῶν" κατὰ τὸ ἑξῆς ἀπεσχημένους ἀφροδισίου ἡδονῆς προσέταξεν ἐπακούειν τῶν θείων λόγων.

It was, therefore, reasonable when Moses in his attempt to bring the Jews to continence by degrees, directed that after sexual intercourse they must abstain for three days before they heard the divine words.