Reference for Nedarim 37:8

הניחא לשמואל דאמר דכן מלטמא אחרים אבל טומאת עצמן יש בהן שפיר

in respect of becoming unclean [themselves], are unclean; in respect of defiling others, they are clean:<span class="x" onmousemove="('comment',' They do not defile them. ');">⁸</span>

Jastrow