Tosefta zu Könige I 5:35

Rabbi Shimon ben Mensya says: "Song of Songs defiles the hands, because it is written with divine spirit. And Qoheleth does not defile the hadns for it is only written through the wisdom of Solomon." They said to him, "But did he write that by himself? Look, it says (I Kings 5:12) 'And he said three thousand proverbs,' and it say (Proverbs 30:6) 'Do not add to His words!'"