Sukkah 25B

רבי עקיבא אומר: מישאל ואלצפן היו, שהיו עוסקין בנדב ואביהוא. רבי יצחק אומר: אם נושאי ארונו של יוסף היו, כבר היו יכולין ליטהר. אם מישאל ואלצפן היו, יכולין היו ליטהר.

R' Akiva says: They were Mishael and Elzaphan, who were engaged in carrying the bodies of Nadav and Avihu after they were burned in the Holy of Holies (see Leviticus 10:4). R' Yitzḥak says: These identifications are inaccurate, because if they were the bearers of Joseph’s coffin, they could have already been purified. They were camped at Sinai sufficient time to become purified in time to sacrifice the Paschal lamb. And if they were Mishael and Elzaphan they could have already been purified, as the Tabernacle was erected on the 1st of Nisan, which was the 8th day of the inauguration, when the sons of Aaron were burned. More than 7 days remained until the eve of Passover on the 14th of Nisan.

אלא, עוסקין במת מצוה היו שחל שביעי שלהן להיות בערב פסח, שנאמר: ״ולא יכלו לעשות הפסח ביום ההוא״, ביום ההוא אין יכולין לעשות, הא למחר ⁠— יכולין לעשות!

Rather, they were unnamed people who were engaged in tending to a corpse whose burial is a mitzva, i.e., which has no one else available to bury it, and their 7th day of impurity occurred precisely on the eve of Passover, as it is stated: “And they could not observe the Pesaḥ on that day” (Numbers 9:6). The Talmud infers: On that day they could not observe it; on the next day they could observe it. Although they would be purified at nightfall and would then be eligible to partake of the Paschal lamb, at the time of the slaughter and the sprinkling of the blood they were not yet pure. They asked whether the Paschal lamb could be slaughtered on their behalf. Apparently, they were obligated to perform the mitzva of burial of the corpse although it prevented them from fulfilling the mitzva of sacrificing the Paschal lamb, which is a stringent mitzva. This is the source for the principle that one engaged in the performance of a mitzva is exempt from performing another mitzva.