Reference for Yoma 101:2
ומאי שנא תמורה שם תמורה אחת היא זבח איכא בכור ואיכא מעשר
Why is it different with substitute animals? - The substitutes all have uniform rules, whereas the original sacrificial animal includes first-born and tithe for cattle. Now, as to R'Shesheth, why does he refer the teaching to the ram of Aaron, let him rather refer to the paschal lamb, which suspends the laws of the Sabbath and of levitical uncleanness and can have a substitute because it is an individual's sacrifice? -He holds that a paschal lamb is never offered for one individual.<span class="x" onmousemove="('comment',' This is the view of R. Judah (Pes. 91a) , there being always more than one to subscribe to the cost of the paschal lamb, which must be eaten up within its prescribed limited time, Ex. XII, 10.');"><sup>2</sup></span>