Points derived from the ḥadīth:
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One of the causes that make the prostrations of forgetfulness obligatory is doubt that arises during the prayer. This ḥadīth lays down the ruling for sujūd al-sahw on account of such doubt—provided the misgiving is not a chronic obsessive whisper (waswās) that dogs a person, so that after performing an act he tells himself he has not done it. Ibn Qudāmah said: “None of the Companions suffered from such compulsive misgivings, and had the Prophet ﷺ encountered those who were constantly so afflicted he would have ordered them killed.”
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The ḥadīth shows that when a worshipper is unsure whether, for example, he has prayed two or three rakʿahs, he must dismiss the doubt, rely on what he is certain of—namely the lesser number—and, before the salām, perform the two prostrations of forgetfulness. Al-Nawawī said: “If a person is in doubt and neither possibility outweighs the other, he must build on the lesser number by consensus. This differs from the one whose predominant impression is that he has, for instance, prayed four.” The Shaykh (Ibn Taymiyyah) added: “The well-known position of Aḥmad is that one proceeds according to what he deems more probable, and this principle applies to most matters of the Sharīʿah.”
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The ḥadīth explicitly affirms that the prayer remains valid and that nothing has occurred to invalidate it. This is the opinion of the majority of scholars, including the three imams—Mālik, Aḥmad, and al-Shāfiʿī. Al-Sharḥ reports that some of the Successors held the prayer must be repeated, but the present ḥadīth supports the first view, which judges the prayer sound once it is mended. Al-Qarāfī said in al-Dhakhīrah: “Drawing near to Allah with a patched-up, mended prayer—when doubt has arisen in it—is better than abandoning the repair and beginning a new one; and restricting oneself to the prayer after mending it is better than repeating it, for such was the practice of the Prophet ﷺ.”
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In juristic usage, doubt here signifies anything short of certainty. It therefore embraces ẓann—the weighing of two possibilities when one is weaker—as well as true doubt where the two sides are equal. All of this counts as “doubt” in the eyes of the jurists and obliges one to act on certainty, for the liability of the servant remains engaged by the obligation and is not discharged except with certainty.