משנה על ויקרא 12:2

Rabbi Akiba declared three things; about two they agreed with him, and about one they disagreed with him.About a lime-burner’s sandal, that it is liable to contract midras impurity; And about the remains of a [broken] oven, that they must be four handbreadths high [in order to retain impurity], whereas they used to say three and [when he said four] they agreed with him. And about one they disagreed with him About a stool, from which two of its covering-boards had been removed, the one beside the other, which Rabbi Akiba pronounces able to contract impurity, but the Sages declare unable to contract impurity.