Ohalot, Chapter One, Mishnah Two
1) What [is the case of] three? Vessels touching a corpse and [other] vessels [touching these] vessels are defiled with seven days defilement.
2) The third: whether a person or vessels, is defiled with a defilement lasting until the evening.
Section one: A vessel that comes into contact with a dead body is unclean for seven days. These vessels are “fathers of impurity.” The same is true for vessels that come into contact with these vessels. These too are “fathers of impurity” and are unclean for seven days.
Section two: But the “third” link in the chain, a person or vessel who comes into contact with a vessel that had contact with another vessel that was in contact with a corpse, has a lower degree of impurity. He or it is impure only until the evening with “first degree impurity.”