Rashi on Genesis 18:8/רש"י על בראשית י״ח:ח׳

1ויקח חמאה וגו AND HE TOOK חמאה, ETC. — but he did not bring bread, on account of Sarah becoming mentstruant; as the 'way of women' returned to her that day, and the dough became impure (Bava Metzia 87a).