What is the ruling for someone who deliberately omits a Sunnah of the obligatory prayer and does not perform the prostration of forgetfulness (sujud al-sahw), and is their prayer valid or invalid? And what is the ruling when the Imam deliberately omits a Sunnah of the prayer and does not perform the prostration of forgetfulness?

Scholars have differed regarding the ruling on the prostration of forgetfulness (sujud al-sahw). The Hanbalis divide the actions of prayer into pillars (arkān), obligatory acts (wājibāt), and sunnah acts. Their madhhab is that whoever omits a prostration of forgetfulness that arose from omitting an obligatory act of prayer has their prayer invalidated. The Hanafis' view is similar to theirs.

According to the Malikis, prayer is invalidated if one intentionally or forgetfully omits a prostration of forgetfulness that arose from omitting three or more sunnah acts, provided that the prostration is not made up before a long time passes or before leaving the mosque.

According to the Shafi'is, the actions of prayer other than the pillars are of two types: abʿād and non-abʿād. The abʿād are, for example, the first tashahhud and sitting for it, the qunut and standing for it. Each of these abʿād, if omitted forgetfully or intentionally, is compensated for by the prostration of forgetfulness. As for the non-abʿād, one does not prostrate for them, whether they are omitted intentionally or forgetfully.

There is no difference in this ruling between an imam who intentionally omits the prostration of forgetfulness and a person praying alone. As for the congregant (ma’mum), if they forget behind the imam, there is no prostration of forgetfulness upon them for acts other than the pillars.