Abu Hanifah makes setting out for Jumuah the decisive point; Abu Yusuf and Muhammad make actually joining the imam decisive. Mere intention, physically leaving, and entering the prayer must not be treated as identical acts.
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The distinction between proceeding and joining
Friday Prayer: Conditions, Khutbah, Congregation and Related Rulings
Abu Hanifah makes setting out for Jumuah the decisive point; Abu Yusuf and Muhammad make actually joining the imam decisive. Mere intention, physically leaving, and entering the prayer must not be treated as identical acts.