What is the ruling on an Imam's lapse if he doubts the number of rak'ahs, but the congregants are unaware of his doubt, and the prayer is completed with all its pillars?

If the Imam doubts the number of Rak'ahs, he should build upon the lesser number and complete his prayer, then perform the two prostrations of forgetfulness (Sujud al-Sahw) before the Taslim. The congregants' unawareness of the Imam's doubt does not affect the validity of the prayer or the permissibility of Sujud al-Sahw.