The time for the removal of leaven is juxtaposed to the time when the eating of leavened bread is prohibited. When the prohibition against eating leaven goes into effect, the obligation to remove leaven is in effect as well. And furthermore, the time of the prohibition against the eating of leavened bread is juxtaposed to the time for the eating of matza, as its prohibition takes effect from the time that the mitzva to eat matza takes effect.
השבתת שאור לאכילת חמץ, דכתיב: ״שבעת ימים שאר לא ימצא בבתיכם כי כל אכל מחמצת ונכרתה״.
The Talmud elaborates: The removal of leaven is juxtaposed to the eating of leavened bread, as they appear in the same verse, as it is written: “7 days leaven should not be found in your houses, as anyone who eats that which is leavened, that soul should be cut off from the congregation of Israel” (Exodus 12:19).
ואכילת חמץ לאכילת מצה, דכתיב: ״כל מחמצת לא תאכלו בכל מושבתיכם תאכלו מצות (וגו׳)״, וכתיב ביה במצה: ״בערב תאכלו מצות״.
And the prohibition against the eating of leavened bread is juxtaposed to the eating of matza, as both appear in the same verse, as it is written: “You should not eat anything that is leavened; in all of your dwellings you should eat matzot, etc.” (Exodus 12:20), and it is written with regard to matza: “On the 1st day, on the 14th day in the evening you should eat matzot” (Exodus 12:18). Since the halakha that leaven is prohibited on the first night of Passover is derived from this source, there is no need for an additional derivation.
ואימא לרבות ליל ארבעה עשר לביעור? ״ביום״ כתיב.
The Talmud asks: And say that the verse: “Yet on the 1st day you should remove leaven from your houses” comes to include the night of the 14th in the obligation to remove leaven, which would mean that one must remove all leaven from his house on the night of the 14th. The Talmud rejects this suggestion: That is not possible, as “on the day” is written.