Wa alaikum assalam, If this was in the final tashahud, you prayer is invalid. If is was in the first one, you should offer sujood Assahu.

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Wa alaikum assalam, It is mandatory to cover the shoulders but if one doesn’t, he is sinful but the prayer is valid.

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Wa alaikum salaam, One must not leave such a great habit and he’d lose the immense reward of such Sunnah but it is not mandatory.

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