Pesachim 79B

וטמאין אין עושין לא את הראשון ולא את השני. בראשון לא עבדי ⁠— דלא הוו רובא. בשני לא עבדי ⁠— דלא הוו מיעוטא.

and those who are impure do not perform the ritual of the Paschal lamb on the first Pesaḥ or the second. They do not perform the ritual of the Paschal lamb on the first Pesaḥ because they are not the majority, and the Paschal lamb may be sacrificed in a state of impurity only when the majority of the community is impure. Additionally, they may not perform the ritual of the Paschal lamb on the second Pesaḥ because they are not the minority, and only the sacrifice of a minority of the community is deferred to the second Pesaḥ.

תנן: נטמא קהל או רובו, או שהיו הכהנים טמאין והקהל טהורים ⁠— יעשה בטומאה. רובו הוא דעבדי בטומאה, אבל פלגא ופלגא לא עבדי בראשון. קשיא לרב!

The Talmud raises an objection from that which we learned in the Mishnah: If the entire community became ritually impure, or if most of it became impure, or if the priests were impure and the community was pure, they should perform the ritual of the Paschal lamb in impurity. This indicates that it is only when most of the community is impure that they perform the ritual of the Paschal lamb in impurity, but if it is half and half, they do not perform the ritual of the Paschal lamb on the first Pesaḥ. This poses a difficulty to the opinion of Rav.

אמר לך רב: רובא עבדי כולהו בטומאה, פלגא ופלגא ⁠— הללו עושין לעצמן והללו עושין לעצמן.

Rav could have said to you: When a majority of the community is impure, they may all perform the ritual of the Paschal lamb in impurity. Even those who are still pure are not required to ensure that they remain pure in order to sacrifice the Paschal lamb. When it is half and half, these who are pure perform the ritual of the Paschal lamb for themselves in a state of purity and these who are impure perform the ritual of the Paschal lamb for themselves in a state of impurity.