What should a person do if he sits down in the third rak’ah of a four rak’ah prayer by mistake?

I sat down in the third rak’ah of prayer by mistake and I remembered my mistake while reciting the prayer of Rabbana; What should I do in such a situation? I performed sajdah as-sahw and recited the same duas after saluting to the right and finished reciting the prayer of Rabbana and finished the prayer, is my prayer valid or should I perform it again? If I should repeat it, should I need to perform its sunnah as well?

Dear Brother / Sister,

When a person sits down for tashahhud in the third rak’a of a prayer of a four-rak’ah prayer by mistake, he stands up and continues his prayer when he realizes this, completes his rak’ahs and performs sajdah as-sahw (prostration of forgetfulness) at the end of the prayer because the qiyam, (the Standing Position in the Salat) which is one of the fards of prayer, is postponed.

Postponing a fard of prayer, not performing one of the wajibs by forgetting or postponing them or performing beforehand necessitate sajdah as-sahw.

It is necessary for you to perform that four rak’ah prayer again since there is a missing rak’ah in your prayer.

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