What distinguishes the prayer of a Muslim from the prayer of an unbeliever?

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Prayer is established as proof of Islam because it consists of specific actions that distinguish the Muslim from the disbeliever. These distinctive actions include facing the Qiblah, bowing (Ruku'), and prostrating (Sujud). Mere standing is insufficient, as disbelievers also stand in their own worship. This distinction between the Muslim's prayer and the disbeliever's prayer is what allows the prayer to establish Islam, unlike fasting which is an internal intention, or Zakat which is analogous to general charity.