What is the ruling when an Imam who leads the prayer becomes unsure of the number of Rak'ahs performed?
Chapter on Praying with Impurity and Other Matters
Al-Mughni
Book of Prayer
Primary text
If the substitute Imam is unsure of the number of Rak'ahs performed, the ruling suggests building upon certainty. If the congregation agrees with the Imam's assumption of certainty, the prayer continues; otherwise, they should inform him (subbah). Upon being informed, he should return to leading them and perform the prostration of forgetfulness (Sujud as-Sahw). The basis for this is that uncertainty regarding the number of Rak'ahs does not permit the appointment of a substitute Imam, similar to a non-substitute prayer leader. Furthermore, the ruling to rely on certainty applies because the Imam is someone without a strong presumption (Zann), thus necessitating adherence to certainty, just like all other worshippers.
Supporting text
Al-Nakh'i opined that the Imam should consider what those praying behind him are doing. Al-Shafi'i stated that the Imam should act based on his own assessment; if they alert him, he should sit, assuming the alerted number is the fourth Rak'ah. Al-Awza'i ruled that the Imam should pray one more Rak'ah with them because he is certain that one Rak'ah remains, then he should step back and advance someone else to lead them in the remainder of their prayer, after which the selected person stands to complete their own prayer. Malik stated that the Imam should pray a complete prayer for himself, and if they finish their prayer, they should sit and wait for him.