Prayer: Does Someone Who Joins in the Third Rak‘ah but Mistakenly Doesn’t Recite Need to Repeat the Prayer?

Practical Laws of Islam as per the teachings of Imam Khamenei

Prayer: Does Someone Who Joins in the Third Rak‘ah but Mistakenly Doesn’t Recite Need to Repeat the Prayer?

English:

Question #577:

A person joins a congregational prayer in the third rak‘ah. However, he does not read anything, thinking that the imam is in the first. Does this person have to repeat the prayer?

Answer #577:

If he realises this before rukū‘, then it is obligatory for him to recite the Fātiḥah and another chapter. However, if there is no time for both, he should recite the Fātiḥah only and join the imam in his rukū. If he happens to realize it in rukū‘ or later, then the prayer is correct and there is no need to do anything. Even though, according to the mustaḥabb caution, he may perform two prostrations of inadvertence for missing out the recitation unintentionally.

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