What happens if a pillar, such as a prostration or bowing, is forgotten and remembered only after starting the recitation of the subsequent Rak'ah?
Chapter on the Two Prostrations of Forgetfulness
Al-Mughni
Book of Prayer
Primary text
If an essential element of a Rak'ah is omitted due to forgetfulness and is only remembered after beginning the recitation of the subsequent Rak'ah, the Rak'ah in which the pillar was omitted is invalidated, and the current Rak'ah becomes the preceding one. Ahmad stated this in the narration of the group, stipulating that if one stands up before initiating the action for the next unit, one should descend and perform the prostration, and it counts for the first Rak'ah. If one has initiated the action for the next unit, the first Rak'ah is nullified, and the current one becomes the first. In this case, the initial opening supplication (Istiftah) stands and does not need repetition.
Supporting text
Al-Shafi'i states that if the omitted pillar is remembered before the second prostration of the following Rak'ah, one returns to the first prostration. If remembered after completing the second prostration, both prostrations count for the first Rak'ah because its action was valid, and the subsequent action due to forgetfulness does not invalidate the first. Malik holds that if a prostration is missed and remembered before raising the head from the bowing of the second Rak'ah, the first Rak'ah is nullified. Al-Hasan, An-Nakha'i, and Al-Awza'i maintain that one performs the missed prostration whenever it is remembered during the prayer. Ahl ar-Ra'y hold that if four prostrations from four Rak'ahs are missed and remembered during the Tashahhud, one performs four prostrations immediately, and the prayer is complete.