Prayer: What Is the Ruling if Someone Mistakenly Prays Behind a Follower Instead of the Imam?

Practical Laws of Islam as per the teachings of Imam Khamenei

Prayer: What Is the Ruling if Someone Mistakenly Prays Behind a Follower Instead of the Imam?

English:

Question #572:

There are two people and between them they select one as the imam and the other follows him in the prayer. A third person enters and starts his prayers behind the one who is following thinking that he is the imam. As soon as the prayer finishes, the third person realizes that the person was not the imam but in fact he was following the imam. What is the rule regarding the prayer of the third person?

Answer #572:

To start one’s prayer behind someone who is himself following an Imam is not correct. However, if one starts his prayer behind such a person due to ignorance but regarding rukū‘ and prostration he fulfills all requirements of one who prays individually, i.e. without increasing or decreasing a rukn intentionally or unintentionally, the prayer is considered correct and valid.

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