What is the ruling if one omits the first Tashahhud and remembers it only after beginning the recitation (Qira'ah) in the subsequent standing rak'ah?
Chapter on the Two Prostrations of Forgetfulness
Al-Mughni
Book of Prayer
Primary text
If a person remembers the omitted first Tashahhud after commencing the recitation in the standing position, he must proceed with his prayer without returning to the sitting position. This is the opinion of the majority of scholars, including 'Umar, Sa'd ibn Abi Waqqas, Ibn Mas'ud, Mughirah ibn Shu'bah, An-Nu'man ibn Bashir, Ibn az-Zubayr, Ad-Dahhak ibn Qays, and 'Uqbah ibn 'Amir. The supporting principle is that once a person has begun an intended pillar of the prayer, he is not permitted to return, similar to commencing the bowing (Ruku'). This is supported by the hadith of Mughirah.
Supporting text
Al-Hasan stated that he may return unless he has started bowing (Ruku'). This view is considered incorrect due to the hadith of Mughirah. Mu'awiyah once stood up in the second rak'ah, and when reminded, he refused to sit until finishing the prayer, then performed two prostrations while sitting, citing that he saw the Messenger of Allah do this.