How should one remedy the omission of an obligatory pillar (Rukn) committed forgetfully?
Chapter on what invalidates the prayer if omitted intentionally or forgetfully
Al-Mughni
Book of Prayer
Primary text
If an obligatory pillar is omitted forgetfully, and the person remembers it during the prayer, they must perform the omitted act. If they only remember after completing the prayer, the ruling depends on the duration of the separation (fasl). If the separation is brief according to custom and usage, they build upon the remainder of their prayer. If the separation is prolonged, they must restart the entire prayer. Imam Ahmad affirmed this ruling, which aligns with the view of Al-Shafi'i and a similar position held by Malik. The correct opinion asserts that the length of separation is determined by customary understanding, not by arbitrary legal quantification.
Supporting text
A faction of our associates holds that if a pillar is omitted and the person reaches the final salutation without recalling it, the prayer is invalidated. Some Shafi'i scholars specify that the long separation is equivalent to the duration of one rak'ah, while others set it as the duration of the prayer being repeated. If one forgets a prostration from the fourth rak'ah, greets the taslim, and speaks words related to prayer, they must complete one rak'ah, then perform the tashahhud, offer the salutation, and follow it with the prostrations of forgetfulness (sujud as-sahw). If the speech was unrelated to prayer, the prayer must be restarted.