Negative commandment · Lo Ta’aseh #115 of 613
Not to slaughter the Paschal sacrifice while one possesses chametz
שלא ישחוט הפסח והחמץ קיים
The verse · Shemot 34:25
לֹא־תִשְׁחַט עַל־חָמֵץ דַּם־זִבְחִי וְלֹא־יָלִין לַבֹּקֶר זֶבַח חַג הַפָּסַח
“Thou shalt not offer the blood of my sacrifice with leaven; neither shall the sacrifice of the feast of the passover be left to the morning.”
The Rambam’s words
Not to slaughter the Paschal sacrifice while one possesses chametz, as [Exodus 23:18] states: "Do not sacrifice the blood of My offering in the presence of chametz."
שלא ישחוט הפסח והחמץ קיים, שנאמר "לא תשחט על חמץ, דם זבחי" (שמות לד,כה).
Why · Sefer HaChinuch #89
That we not slaughter the lamb of the Pesach-offering on the fourteenth of Nissan while chametz is still in our possession
“It is from the roots of the commandment [that it is] because it is something known to every man that setting the time for all matters is the preservation of their doing.”
Who & when
“And [it] is practiced in the time of the Temple by males and by females.”
Where in the Mishneh Torah
Hilchot Paschal Offering הלכות קרבן פסח