What is required if one omits the Tashahhud (sitting for the second attestation) and realizes it later?
Chapter on the Two Prostrations of Forgetfulness
Al-Mughni
Book of Prayer
Primary text
If the Tashahhud is omitted, one should perform one prostration to substitute for the missed rak'ah, and then complete three more rak'ahs. In one narration from Abu Abdullah (Imam Ja'far al-Sadiq), he stated that this individual is deemed to have been playing and must restart the prayer entirely. This scenario is based on the rule that if one forgets a pillar (rukn) of a rak'ah and only recalls it upon starting the recitation of the next rak'ah, the previous rak'ah is invalidated. Thus, if the first prostration of forgetfulness is missed upon entering the second rak'ah, the first rak'ah is voided. Similarly, the second rak'ah is voided upon starting the third, and the third upon starting the fourth. Only the fourth rak'ah remains, and one prostration is performed for it, requiring two additional rak'ahs to complete the count, summing to three additional rak'ahs prayed after the initial missed unit.
Supporting text
Another narration suggests the entire prayer must be restarted, viewing the situation as tampering with the prayer due to the numerous invalidated components. Shafi'i suggested two rak'ahs would suffice, as the initial move to the second rak'ah was erroneous and void, and the subsequent prostration would fulfill the first missed unit, completing the count in stages.