Q 4: How can an Imam who increased or shortened his prayer, and did not know until after Taslim (salutation of peace ending the Prayer) perform Sujud-ul-Sahw (Prostration of Forgetfulness)? And how can a late comer to prayer perform Sujud-ul-Sahw after making up the missed parts of prayer? Shall he prostrate with the Imam when he prostrates [for forgetfulness] or not?

A: Sujud-ul-Sahw is obligatory when forgetting anything that renders the prayer Batil (null and void) and it is performed before saying Taslim as reported by Abu Sa`id, `Abdullah ibn Buhaynah and others. However, in case of forgetting a Rak`ah (unit of prayer) or more, whether forgetfulness is ascertained or based on strong probability, the missed part should be made up for after saying Taslim. The Hadith narrated by Abu Hurayrah and `Imran ibn Husayn in regard to the story of Dhul-Yadayn gives evidence to this effect, in addition to the Hadith narrated by Ibn Mas`ud concerning offering what is thought to be missed.Therefore, this Imam who increased in his prayers and did not know except after Taslim should perform a prostration of forgetfulness then make Taslim. But if he forgets a Rak`ah or more, then he should perform it then make Taslim then he perform a prostration of forgetfulness then make Taslim. (Part No. 7; Page No. 128) If the follower is able to perform prostration of forgetfulness with the Imam before making up what he has missed, then let him do it. However, if he stands up to complete his prayer before the Imam prostrates for forgetfulness, then he is separated from the Imam and he does not prostrate with him. After he completes what he has missed, he has to prostrate for forgetfulness as mentioned before.