What they are doing is wrong. They should wait until he finishes both tasleem.

I see some people who join congregation late get up to peform the missed rakahs as soon as the Imam turns right to say Assalamu alaikum wa rahmatullah. They don’t wait for the Imam to turn left and say Assalamu alaikum… Is it correct?

Prayer | March 26, 2014 9:44 am
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I have hurt my colateral ligament (the bone behind the has twisted in the injury) and i can not sit in tashahud becaus my knee hurts, untill i get well is it a sin to sit on a chair only in tashahud.becaus some say to me that i have to sit just like the prophet Muhammed (May Allahs Peace and Blessing Be Upon Him) did and some say that you can use a chair if you are unable to sit or becaus of and injury, so that is my question: if i can sit on a chair only for tashahud?
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And is it true that anyone who misses his Asr prayer will have all his good deeds (including his fasts, prayers etc) cancelled?