Chabad.org — English translation + Rashi
Torah Temimah points to Zevachim 8b.
E-Daf.com — Zevachim 8b
HebrewBooks / Shas PDF
English: A pesach not in its time that was slaughtered not for its own sake is valid, as it is written “and if from the flock is his offering for a sacrifice of shelamim” — something that comes from the flock as shelamim.
Pesach outside its time as shelamim — an out-of-season pesach slaughtered shelo lishmah is treated as valid shelamim.
An out-of-season pesach slaughtered shelo lishmah is treated as valid shelamim. The Gemara reads the verse’s description of a flock animal brought “for a sacrifice of shelamim” as supplying the legal identity that such a pesach acquires. The result is that the animal is no longer disqualified by its original pesach designation once the time for pesach has passed and the slaughter was not for that purpose.