Prayer: If I Skip a Rak‘ah and Realise in the Last Rak‘ah, How Do I Correct the Prayer?

Practical Laws of Islam as per the teachings of Imam Khamenei

Prayer: If I Skip a Rak‘ah and Realise in the Last Rak‘ah, How Do I Correct the Prayer?

English:

Question #519:

What is the duty of a person who forgets to perform a rak‘ah of his prayer and then remembers it in the last rak‘ah? For example, thinking first rak‘ah of his prayer to be the second one, he performs the third and the fourth rak‘ah; then in this last rak‘ah he realises that it is really only the third rak‘ah. What should he do?

Answer #519:

It is obligatory for him before reciting the final salām of the prayer to perform the rak‘ah he has missed and then recite the salām. As he had not done an obligatory tashahhud in its proper place, it is obligatory for him to perform the two prostrations of inadvertence for the forgotten tashahhud and, based on caution, to make up for it in qaḍā’ before the two prostrations of inadvertence. Moreover, if he says salām in the third rak`ah, thinking that it is the last rak`ah, it is an obligatory caution to perform another two prostrations of inadvertence which should be done after the qaḍā’ of the tashahhud.

-Imam Khamenei, Practical Laws of Islam, Importance and Conditions of Prayer