A man was leading people in a four-Rak`ah Salah (Prayer consisting of four units), and instead of sitting to recite the Tashahhud (a recitation in the sitting position in the last unit of Prayer) in the fourth Rak`ah, he stood up.
Some of the Ma'mums (those being led by an Imam in Prayer) said, "Subhan Allah (Glory be to Allah)" to remind him, so he sat down after he had been standing. Some scholars said that his Salah (Prayer) was nullified, because he returned to sitting after he had been standing. Others said that his Salah was valid, as he just moved from one Rukn (Pillar) to another Rukn of Salah, not from a Rukn to a Sunnah act of Salah, as would have been the case if he has sat down from standing to perform the first Tashahhud; that would have made his Salah invalid, but what he did made his Salah valid. What is the ruling on this Salah and what is the evidence for it?
The Salah that you mentioned is valid, and it would not have been permissible for him to do other than he did. By his standing he was adding to the Salah something, which if done intentionally would be invalid. Adding to Salah is not permissible if the person knows about it or they are reminded by someone, according to the Ijma` (consensus of scholars). As to sitting to recite the first Tashahhud (Part No. 7; Page No. 137) after standing, this does not nullify the Salah, but it is better not to sit after standing upright and starting a recitation of the Qur'an. In this case a Sujud-ul-Sahw (Prostration of Forgetfulness) should be performed.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.