Prayer: Following an Imam Performing a Qadaʾ Prayer
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1389
One can follow the imam of a congregational prayer who is performing the qaḍāʾ of his own or someone else’s daily prayer about which he is certain. However, if he is performing such a qaḍāʾ prayer as a precautionary measure [as opposed to being certain about it being a qaḍāʾ prayer], then it is not permitted to follow him unless the follower is also performing his prayer as a precautionary measure and the reason for his precaution is the same as that of the imam’s;[1] and if the follower has another reason for his precaution as well, he can still follow.
-Ayatullah Sistani, Practical Laws of Islam, Prayer (Salah)
For example, the imam and the follower have deemed it probable that the place where they both performed a previous prayer was usurped, and so they are now performing that prayer again as a precautionary measure.