Rashi on Deuteronomy 23:12/רש"י על דברים כ״ג:י״ב

1והיה לפנות ערב BUT IT SHALL BE WHEN EVENING COMETH ON, [HE SHALL LAVE HIMSELF WITH WATER] — He should immerse himself close before the setting of the sun, for under no circumstances is he clean without having waited for the sunset (cf. Sifrei Devarim 256:2).