Quotation for Pesachim 239:5

שבעת ימים מצות תאכלו SEVEN DAYS SHALL YE EAT UNLEAVENED BREAD — But in another passage it slates: (Deuteronomy 16:8) “Six days shalt thou eat unleavened bread”! This teaches regarding the seventh day of the Passover that it is not obligatory to eat unleavened bread on it, but only this is required viz., that one should not eat leavened food. Whence may we derive that the other six days, too, are optinal as regards the eating of unleavened bread? From what Scripture states: “Six days [shalt thou eat unnleavened bread]”. For the following is a Rule by which the Torah may be expounded: Anything that is included in a general statement and goes out (i. e. Scripture singles it out) from this general statement for the purpose of teaching something, does not go out from it in order to teach something regarding itself alone but it goes out from it to teach something about everything that is included in the general statement. Now the seventh day is included in the general statement “Seven days shall ye eat unleavened bread”, and in the text “Six days shall thou eat unleavened bread” it has left the general statement. How is it with the seventh day? It is optional as regards the eating of unleavened bread, (as explained in the earlier portion of this comment)! This, according to the above rule, applies also to everything that was included in the general statement i. e. to the whole seven days, and therefore the other six days are also optional in this respect! One might think that the first night of the Passover is also optional (since it is part of the first day when as we have just shown the eating of unleavened bread is optional), therefore Scripture states, (v. 18) “at evening ye shall eat unleavened bread” Scripture thus fixes it as obligatory for that night (Pesachim 120a).