Rashi on Leviticus 15:12/רש"י על ויקרא ט״ו:י״ב

1וכלי חרס אשר יגע בו הזב AND THE VESSEL OF EARTH THAT HE WHO HATH THE ISSUE TOUCHETH [SHALL BE BROKEN] — One might think that the law applies also if he touched it from outside (its exterior) etc., as it is stated in Torath Cohanim (Sifra, Metzora Parashat Zavim, Section 3 1).