Rashi on Deuteronomy 6:20/רש"י על דברים ו׳:כ׳

1כי ישאלך בנך מחר WHEN THY SON ASKETH THEE — There is a usage of the word מחר that refers to a day that only comes after the lapse of some time (and this is so here, i.e. it here means “in time to come", not "to-morrow”) (Mekhilta).