Those who write in figh and the ahaadeeth of the legal rulings began their books with at-tahaarah because it is the key to the prayer which is the most significant of the pillars of Islaam after Ash-shahaadatain, bearing witness that there is no deity that has the right to be worshipped except Allaah and that Muhammad is the Messenger of Allaah. So there is no prayer without purification.