Mischna zu Bamidbar 19:21

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.