Completes the missed part of the Prayer before Imaam's tasleem
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Is it permissible for the latecomer to (join the congregational prayer and then in the final tashahhud) stand and complete what he has missed of the prayer before the imaam has actually completed the (congregational) prayer?
This is not permissible for him to do; He must remain seated until the imaam has completed (the prayer with) the second tasleem. And if he were to stand before the tasleem and does not (immediately) return (to join the imaam), then his prayer is (automatically) rendered a voluntary prayer and he is required to repeat the (obligatory) prayer; This is because the congregation (behind the imaam) are obligated to remain with the imaam (and follow him) until the imaam completes the prayer.
Shaykh 'Abdur-Rahmaan as-Sa'dee
al-Fataawa as-Sa'diyyah - Page 131, Question 29