Commentary for Pesachim 76:14

You shall guard the Matzot, guard it for the purpose of Matza - every time you guard it from becoming leaven, you should have in mind the mitzva of matza.

"which cannot be eaten for seven day" - since after a day and a night it becomes "notar" as the verse states "And the flesh of his thanksgiving sacrifice of well-being shall be eaten on the day that it is offered; none of it shall be set aside until morning" (Vayikra 7:15). In a case where the sacrifice did not end up being slaughtered, the matzot may now be eaten for seven days. However, they were not intended to be the mitzva of matza but were intended to become unfit if they were not used along with the sacrifice within the timeframe of a day and a night. Rav Yosef agrees with the teaching of Rabba who requires the Matza be guarded for the sake of the mitzva and relies on the same verse as Rabba. He only argues with Rabba on this point that if he also had intention for the mitzva of matza then he would fulfill the requirement of the thanksgiving sacrifice but would not fulfill the requirement of matza that requires guarding matza that can be used for seven days.