Reference for Pesachim 33:7
תא שמע (חגי ב, יג) ויאמר חגי אם יגע טמא נפש בכל אלה היטמא ויענו הכהנים ויאמרו יטמא בשלמא לשמואל מדהכא לא אישתבש התם נמי לא אישתבש אלא לרב מאי שנא הכא דאישתבש ומאי שנא התם דלא אישתבש
And the priests answered and said, It shall be unclean.<span class="x" onmousemove="('comment',' Hag. II, 13,');"><sup>7</sup></span> As for Samuel, it is well: since they did not err here, they did not err there [either]; but according to Rab, why did they err here yet did not err there? - Said R'Nahman in Rabbah B'Abbuha's name: They were well-versed in the uncleanness of a corpse, but not well-versed in the uncleanness of a sherez.<span class="x" onmousemove="('comment',' Thinking that where the originating uncleanness is a sherez, it does not go beyond the third degree.');"><sup>8</sup></span>