Prayer: Joining Without Knowing Which Rakʿah It Is
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1426
If the imam is standing and the follower does not know which rakʿah it is, he can join the congregational prayer; and based on obligatory precaution, he must recite Sūrat al-Ḥamd and the other surah; however, he must recite them with the intention of qurbah.