הלכה אויף דברים 16:4

It is forbidden to derive benefit [from] chamets on Passover. As it is stated (Exodus 13:3), "no chamets shall be eaten" - there shall be no dispensation for eating it. And one who leaves chamets in his domain on Passover - even though he did not eat it - transgresses two commandments, as it is stated (Exodus 13:7; Deuteronomy 16:4) "leaven shall not be seen in all of your border"; and as it is stated (Exodus 12:19) "leaven shall not be found in your houses." And the prohibition of chamets and the prohibition of leaven with which we rise [foods] is the same.

Hence if he did not nullify [it] before six [hours]; and he found chametz from six hours and onward that he had his mind upon and he forgot it at the time of the destruction and did not destroy it - this one has surely transgressed, "it shall not be seen," and "it shall not be found." For he surely did not destroy [it], nor did he nullify [it]. And nullification now does not help him at all, since [the chametz] is not in his possession. Yet Scripture made it as if it was in his possession, to make him liable for "it shall not be seen," and "it shall not be found." So he is obligated to destroy at any time that he finds it. However if he found in on the holiday, he covers it with a vessel until the evening and destroys it [then]. And if it is of consecrated [foods], he does not need to cover it with a vessel, since all separate from it [regardless].

[In a case of] one who left a kneaded dough in his house [on the festival] and went out and remembered [it] after he left; and he is [now] sitting before his teacher and is afraid lest it rise before he comes back: Such a one should surely nullify it in his heart before it leavens. But if it [already] leavened, the nullification does not help at all. Rather, he has already transgressed "it shall not be seen," and "it shall not be found." And he is obligated to destroy it immediately when he returns to his house.