Reference for Zevachim 196:4
אם טבח הוא תיפוק לי משום רבב ואי מוכר רבב הוא תיפוק לי משום דם לא צריכא דעביד הא והא אחדא לא קפיד אתרתי קפיד או דלמא אתרתי נמי לא קפיד תיקו:
Subsequently he solved it, that it does not require washing. Raba said: It is clear to me that blood on his garment interposes, but if [its owner] is a slaughterer, it d not interpose.<span class="x" onmousemove="('comment',' An unclean garment must be immersed in a ritual bath (mikweh) for purification; the ceremony is called immersion. Now, when immersion is performed, no foreign matter may interpose between the article to be purified and the water. Normally, blood is foreign matter, for a person objects to blood on his garment, and it interposes (rendering immersion ineffectual) . A slaughterer, however, does not object to blood on his garment, and so it is not regarded as foreign matter and does not interpose.');"><sup>6</sup></span>