פּירוש אויף שמות 34:25

לא תשחט וגו׳ THOU SHALT NOT OFFER [THE BLOOD OF MY SACRIFICE TOGETHER WITH LEAVEN] — i. e. thou shalt not kill the Passover lamb whilst leaven is still existent in your possession. This is an admonition addressed to him who slaughters the Passover lamb, as well as to him who sprinkles its blood or to one of the members of the party formed to eat that lamb in company (Pesachim 63a).

לא תשחט על חמץ דם זבחי, “Do not slaughter (offer) the blood of My sacrifice with leavened bread;” Rashi explains that this verse is a warning addressed to him who slaughters the Passover lamb, as well as to him who sprinkles its blood or to any of the people invited to eat from that lamb. Compare the baraitha in Pessachim 63. It is stated there that this prohibition is one of the negative commandments. When is that commandment applicable? As long as either the person slaughtering the lamb, sprinkling its blood, or a guest invited to partake of it, still has leavened products in his possession on Passover eve. It had to be inserted here as previously, in 23,18, this had not been spelled out. Here it is called: “the meat offering of the Passover festival.”

NEITHER SHALL THE SACRIFICE OF THE FEAST OF THE PASSOVER BE LEFT UNTO THE MORNING. In line with the plain meaning of Scripture this admonition applies to the whole [Passover-] offering, [the sacrificial portions burnt on the altar, and the flesh eaten by the Passover celebrants]: that one should not leave over of the flesh unto the morning, for that which remains of it until the morning shall be burnt with fire,⁵³¹Above, 12:10. and also the fat of it should be burnt on the altar only until the morning. Onkelos, however, translated the verse as applying [only] to the fat which had to be taken up to be burnt on the altar,⁵³²Thus Onkelos translated: “There shall not remain [overnight] away from the altar till the morning the fat of the sacrifice of the feast of the Passover.” for this “feast” [mentioned there] is explained there, neither shall the fat of My feast remain all night until the morning,⁵³³Above, 23:18. since all these commandments [mentioned here] are based upon those cited there.
Now Rashi wrote: “Neither shall [the sacrifice of the feast of the Passover] be left unto the morning. This is to be understood as the Targum renders it [namely, that it refers to the fat, as explained above]. An offering left overnight is not invalidated if it was placed on top of the altar [during the night, even though it was not burnt at night; and it may be burnt on the altar the following day], neither is it invalidated until the dawn of morning [i.e., if at that time it is not on top of the altar, it is deemed to have been ‘left-over’]. The sacrifice of the feast of the Passover. This refers to the sacrificial portions. From here you derive the law concerning the burning of all fats and limbs of sacrifices [which were burnt on the altar].” In the section of Tzav⁵³⁴Leviticus 6:2. I will explain this with the help of G-d. And with regard to what Rashi wrote about an offering left overnight if it was placed on top of the altar [during the night it is not invalidated] — there is a difference of opinion on this matter in the Gemara,⁵³⁵Zebachim 87a. and in the opinion of Raba it does become invalidated even if on top of the altar; thus if limbs of a sacrifice were left there [on top of the altar] overnight, and then by accident they were taken down [from the altar to the pavement], they may no longer be taken up. On the other hand, if they were not taken down, then even those limbs that had been left over on the pavement of the Court for many days, and by accident were taken up, do not need to come down [and are burnt upon the altar], as is found in the Chapter “The Altar.”⁵³⁵Zebachim 87a.

ולא ילין NEITHER SHALL [THE SACRIFICE OF THE FESTIVAL OF PASSOVER] REMAIN OVER NIGHT — Understand this as the Targum renders it: ולא יביתון ... בר ממדבחא … ‎“There shall not remain overnight away from the altar till the morning the fat of the sacrifice” — this law of leaving the fat away from the altar overnight does not serve to invaliditate the sacrifice if it has been placed on the top of the altar some time during the night, even if it be not entirely burnt by morning, nor is the law about leaving the fat overnight infringed except if it has not been placed on the altar by the dawn of the morning (but any time during the night one may lift it up from the pavement on to the altar (Zevachim 87a; cf. Rashi on Exodus 23:18).

זבח חג הפסח means the pieces of its fat (which had to be burnt on the altar). From here you may derive the law concerning all cases of burning the pieces of fat and the limbs of sacrifices (i. e. you may derive the general rule regarding all sacrifices).