Mischna zu Bamidbar 19:18

A vessel of earthenware can protect everything [in it from contracting impurity], according to Beth Hillel. But Beth Shammai says: “It protects only food and liquids and [other] vessels of earthenware.” Beth Hillel said to them: “Why?” Beth Shammai said to them: “Because it is [itself] impure with respect to an ignoramus, and no impure vessel can screen [against impurity].” Beth Hillel said to them: “And did you not pronounce pure the food and liquids inside it?” Beth Shammai said to them: “When we pronounced pure the food and liquids inside it, we pronounced them pure for him [the ignoramus] only, but when you pronounced the vessel pure you pronounced it pure for yourself and for him.” Then Beth Hillel changed their mind and taught according to the opinion of Beth Shammai.

Rabbi Yishmael says: there are three instances of lenient rulings by Beth Shammai and strict rulings by Beth Hillel.The book of Ecclesiastes does not defile the hands, according to the opinion of Beth Shammai; But Beth Hillel say: it defiles the hands. Water of purification which has done its duty: Beth Shammai pronounces it pure, But Beth Hillel pronounces it impure. Black cumin: Beth Shammai pronounces it not liable to become impure, But Beth Hillel pronounces it liable to become impure. So, too, with regard to tithes.