Prayer: The First Row Completing Their Prayer
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1418
If the prayer of all those standing in the first row comes to an end and they do not immediately join the imam for the next prayer, the congregational prayer of those in the next row becomes invalid. In fact, even if they join immediately, the validity of the congregational prayer of those on the next row is problematic [i.e. based on obligatory precaution, it is not valid].