What is the ruling for a follower (*ma'mum*) if they become aware that the Imam omitted the first Tashahhud after the Imam has already risen for the next unit of prayer?

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The followers must proceed with the standing and not return to sit for the first Tashahhud. This position is narrated by Al-Ajurri from Ahmad, and it is the opinion of Malik, Al-Shafi'i, Abu Thawr, and the people of Iraq. The basis for this ruling is the practice of the Prophet, peace be upon him, who, when he omitted the first Tashahhud and rose, the people rose with him. Furthermore, numerous companions who led prayers, upon rising for the second unit, were alerted by those praying behind them (via Tasbih), and some indicated for them to rise, and they complied. Evidence includes the narration of Mughirah ibn Shu'bah, who, after rising for the third unit, indicated to those who called out to him (Tasbih) to stand, and upon finishing, he performed two prostrations of forgetfulness (Sujud as-Sahw) and stated he followed the practice of the Messenger of Allah. Similarly, when 'Umar ibn Al-Khattab erred in sitting during a Tashahhud and those behind him called out Tasbih, he indicated to them to rise. Ibn Mas'ud and 'Uqbah ibn 'Amir also held that the prescribed Sunnah is to rise, not to sit down upon hearing Tasbih for the first Tashahhud.