Reference for Menachot 160:8
תני רבי חייא תודה שנתערבה בתמורתה ומתה אחת מהן חבירתה אין לה תקנה היכי נעביד נקריב לחם בהדה דלמא תמורה היא לא נקריב לחם בהדה דלמא תודה היא
[Is it not obvious? ] - You might think that it applies only there [in the case of the thank-offering] for one might say that he is offering additional thank-offerings,<span class="x" onmousemove="('comment',' Since a man may offer as many thank-offerings as he pleases and at any time.');"><sup>10</sup></span> but not here [in the case of the sin-offering] for one cannot say that he is offering additional sin-offerings;<span class="x" onmousemove="('comment',' A sin-offering cannot be brought at any time as a freewill-offering; accordingly even in the first case where the atonement was made by the first animal the third animal should also be left to die.');"><sup>11</sup></span> we are therefore taught [that so it is too with the sin-offering].