If I make a mistake while performing a prayer and read something wrong or sit after the third rak'ah instead of the fourth one and read at-Tahiyyat, what should I do?

If I make a mistake while performing a prayer and read something wrong or sit after the third rak'ah instead of the fourth one and read at-Tahiyyat, what should I do?

Dear Brother / Sister,

In those cases, you need to perform prostration of forgetfulness (sajdah as-sahw).

A person who sits after the third rak'ah in a four-rak'ah prayer by mistake for tashahhud needs to stand up when he remembers it and complete his prayer; he needs to perform prostration of forgetfulness at the end of the prayer. For, he has delayed qiyam, which is one of the fards of the prayer. 

Delaying a fard, abandoning a wajib by forgetting it, or delaying it or performing it earlier than its determined time in a prayer, necessitate prostration of forgetfulness.

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