Temurah 22B

גופא: אמר רבא: אבודה דלילה לא שמה אבודה. אליבא דמאן? אילימא אליבא דרבנן ⁠— מאי איריא אבודה דלילה? אפילו אבודה דיום נמי, דאמרי רבנן: אבודה בשעת הפרשה רועה.

§ The Talmud discusses the matter itself, which was cited above: Rava said: A sin offering lost at night, when one cannot sacrifice an offering, is not called lost. Provided that the offering is found before morning, then even if the owner designated another sin offering in its stead, the first sin offering is not left to die. The Talmud asks: In accordance with whose opinion is this statement? If we say that it is in accordance with the opinion of the Rabbis, who maintain that a sin offering is not left to die unless the owner achieved atonement with another offering, why does Rava specifically mention that it was lost at night? Even if it was lost during the day it is not left to die, as the Rabbis say: A sin offering that was lost at the time when another animal was designated in its stead, but was found before this second animal was sacrificed, is left to graze until it develops a blemish.

ואלא, אליבא דרבי, עד כאן לא קאמר רבי אלא אבידה דיום, אבל אבידה דלילה ⁠— אפילו רבי מודה דלרעייה אזלא.

Rather, you must say that it is in accordance with the opinion of R' Yehuda HaNasi, who maintains that if a sin offering was lost at the time when another animal was designated in its stead, the first sin offering is left to die even if it was found before the second animal was sacrificed. Rava teaches that R' Yehuda HaNasi states his opinion only with regard to an animal lost during the day, but if the animal was lost only at night, even R' Yehuda HaNasi concedes that it goes to graze and is not left to die.