ANSWER: If this is voluntary prayers, this is valid and you will get only half of the rewards.  But if this is fard prayers, your prayer is invalid as it is a pillar of the validity of prayers that you stand up when able to do so. 

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ANSWER: If you mean he forgot to sit for the 1st tashahhud, stood up for the 3rd rakah and then sat down immediately then this is not permissible since he had already stood up for the pillar. He should have continued the prayer and made sujood as sahu before making salam for missing the 1st […]

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