Rashi on Genesis 6:20/רש"י על בראשית ו׳:כ׳

1מהעוף למינהו OF THE FOWLS AFTER THEIR KIND— those that had kept to their own species and had not corrupted their way (taking the words to mean, “of the fowls that had kept to their own kind”). All these came of their own accord, but only those which the Ark was allowing to pass in did Noah permit to enter (Sanhedrin 108b).