Midrash for Pesachim 97:12

"shall not be seen unto you": Do not see it as being yours, but void it in your heart — whence they ruled: "If one were on his way (on the fourteenth of Nissan) to slaughter his Pesach offering, or to circumcise his son, or to eat the betrothal feast in the house of his father-in-law, and he remembered that he had chametz in his house — if he can return (home) and burn it and return to his mitzvah, he should do so; if not, he should void it in his heart.