ANSWER:

Of course you must sit with the imam and follow him in every step and when he concludes his prayer by giving salam on both sides, you must get up and pray your remaining rakahs.

When I join behind an imam and I was late so he has already done the first rakaah, do I have to only follow him in his tashahhud or should I also stay sitting after his third rakaah which is my second one and do tashahhud by myself?

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