268 – Abu Saʿīd al-Khudrī (d. AH) reported that the Messenger of Allah ﷺ said: “If any one of you becomes uncertain during his prayer and no longer knows whether he has prayed three or four rakʿahs, let him set the doubt aside and proceed on the basis of what he is certain about. Then he should perform two prostrations of forgetfulness before giving the salām. If it turns out that he actually prayed five, those two prostrations will make the prayer even; and if he had already completed the correct number, the prostrations will serve to humble Satan.” Narrated by Muslim (1).

Vocabulary of the ḥadīth
• al-shakk (doubt): The verb shakka fī l-amr yashukku shakkan means “to be in doubt”; doubt is the opposite of certainty. Plural: shukūk. In al-Taʿrīfāt it is defined as vacillation between two mutually exclusive possibilities with neither outweighing the other—this is the definition preferred by the scholars of uṣūl. Jurists, however, define it as hesitation as to whether an act has occurred or not, even if one possibility slightly outweighs the other.
• fa-lyatraḥ (let him dismiss): that is, let him cast aside what he is unsure about and rely only on what he knows with certainty.
• targhīman li’l-shayṭān (a humiliation of Satan): with an open tāʾ and a silent rāʾ; literally “to rub his nose in the dust (raghām),” meaning to disgrace and humble him.
• wa-lyabni ʿalā mā istayqana (let him build upon what he is certain of): The verb banā yabnī bināʾan, plural abniya, literally refers to constructing physical objects such as a house or wall. Metaphorically, as here, it means to rely on what one is certain one has already performed in the prayer, ignoring what remains doubtful.