This issue is referred to some points:
First: Ruling on performing the prostration of forgetfulness in prayer.
The Ḥanafiys said: “The prostration of forgetfulness is obligatory, and one is sinful for leaving it.” The Shāfiʽiys and Mālikiys said: “It is a Sunnah (recommended).” The Ḥanbaliys said: “It is obligatory for what invalidates the prayer if done intentionally, such as ignoring the Takbīr and Tasmīʽ (Samiʽallāhu liman Ḥamidah), but for what does not invalidate it, performing the prostration of forgetfulness is recommended, such as ignoring the supplications of glorification and magnification (Adhkār).”
Second: The prostration of forgetfulness does not make up for pillars, but it makes up for what is below them in rank. The pillar is not made up for except by performing it in the exact prescribed manner.
Third: The type of prayer.
If the prayer is a voluntary (Sunnah) prayer, then the majority of scholars say that there is no difference between the Sunnah and the obligatory (Farḍ) prayer regarding the prostration of forgetfulness, but some of the Shāfiʽiys differed in opinion. It was also reported from Ibn Sīrīn that the prostration of forgetfulness in voluntary prayer is not obligatory. The Mālikiys differentiate between the Sunnah and the obligatory prayers. They counted five things that do not require the prostration of forgetfulness in the Sunnah prayer.
Fourth: The scholars differed on the ruling of forgetting the prostration of forgetfulness.
The majority of the Ḥanafiys and Shāfiʽiys say that if one forgets the prostration of forgetfulness and leaves the mosque, his prayer is valid and there is nothing obligatory upon him. The Ḥanbaliys say: “If he is still in the mosque, he should perform it, but if he leaves the mosque, his prayer is invalid for the concept of separation.” The Mālikiys differentiate between the prostration that is performed before the Salam and the one performed after it. If he forgets the one that should be made after the Salam, there is nothing obligatory upon him. If he forgets the one that should be made before the Salam, then if he is still in the mosque, he should perform it and his prayer is valid, but if he leaves the mosque, his prayer is invalid, and he must repeat it.
Fifth: Taking the time frame of separation into account.
The scholars differed in determining the time frame of separation on which the ruling is based.
One opinion related to the Ḥanbaliys stated that the time frame of separation is determined by remaining in the mosque and leaving it, as we mentioned before. One opinion related to the Shāfiʽiys stated that it depends on the customary practice. If the customary practice considered an act as separation, the prostration of forgetfulness is invalid even if the time of separation is short. If it is not considered a separation, then performing the prostration of forgetfulness is valid even if the time of separation is long. For example, if a person forgets an act in his prayer and sits in the mosque to remember Allāh for an hour, then remembers that he forgot the prostration of forgetfulness, he can perform the prostration of forgetfulness according to the Shāfiʽiy’s opinion. If another person forgets an act in his prayer and leaves to his car, drives it, and remembers after ten minutes that he forgets the prostration of forgetfulness, he cannot perform the prostration of forgetfulness; here separation is determined based on the customary practice. Mālik’s opinion is that if the prostration of forgetfulness is made for an addition in the prayer, he performs it even if the separation time is long, and this is the second opinion that is attributed to ʼAḥmad and the Shāfiʽiys. However, if the prostration of forgetfulness is made for a reduction in the prayer, he performs it as long as the separation time is not long.
The Hanafis consider the time frame of separation to be leaving the place of prayer or leaving the mosque.
For the sake of this Fatwa, I choose the opinion of the Ḥanafiys.
This issue was a matter of debate between a grammarian and a jurist. Imam As-Sarakhsiy said in “Al-Mabsūṭ”: “If a person is not sure regarding performing the prostration of forgetfulness, he investigates further to be certain of it, and he does not perform it. We have previously clarified that the repetition of Sujūd as-Sahw in the same prayer is not valid because if he prostrates for this forgetfulness, he might forget it a second time or a third time which leads to endless repetition. It was reported that Muḥammad, may Allāh have mercy on him, said to his cousin Al-Kisāʼiy: why do not you study jurisprudence with this sharp mind? He replied: whoever masters a field of knowledge, that field of knowledge will lead him to the rest of the fields of knowledge. Then, Muḥammad said: let me ask you a question from jurisprudence, and you give me its answer in reference to the field of Naḥw (Arabic grammar). He replied: what is it? He said: what do you think of a person who forgets the prostration of forgetfulness for an hour. He replied: no prostration of forgetfulness is obligatory upon him. Muḥammad said: which chapter from Naḥw chapters have you driven this answer from? He replied: I have driven it from the chapter of diminution (Taṣghīr) in which a diminutive form is not exposed to further diminution. Upon that, Muḥammad was astonished for Al-Kisāʼiy’s intelligence.”
After all, Allāh knows best.
Fatwa by Dr. Khālid Naṣr