Mishnah Yadayim 3:5: The Debate Over the Biblical Canon
Lawrence H. Schiffman, Texts and Traditions, Ktav, Hoboken 1998, p.119-120.
Rabbi Simeon ben Azzai said, “I have a tradition from the seventy-two elders (of the Sanhedrin) that on the day when Rabbi Eleazar ben Azariah was appointed head of the Academy, it was decided that Song of Songs and Ecclesiastes defile the hands.” Rabbi Akiva said, “God forbid! No one in Israel disputed about Song of Songs, saying that it does not defile the hands. For all of eternity in its entirety is not as worthy as the day on which Song of Songs was given to Israel, for all the Writings are holy, but Song of Songs is the Holy of Holies. And if they disputed at all, they disputed only regarding Ecclesiastes.”
Rabbi Yohanan ben Joshua the son of Rabbi Akiba’s father-in-law said, “As according to Ben Azzai, so did they dispute and so did they determine [that both Song of Songs and Ecclesiastes are included in the canon].”
104. All books which are included in the biblical canon transmit defilement to those who touch them.
105. The School of Shammai takes the view that Ecclesiastes does not defile the hands, which is to say that it is not canonical. The School of Hillel rules that Ecclesiastes does defile the hands and is part of the canon.