The first Tashahhud in the Salah is one of the obligatory parts of it, according to the more correct of the two scholarly views, because the Prophet (peace be upon him) used to do it and said:
Pray as you have seen me praying.
If he omitted it by mistake, he should prostrate due to his forgetting it. If he omitted it deliberately, then his prayer is invalid. A person who omits it by mistake can make up for it by doing the prostration of forgetfulness before Taslim, whether the person was the Imam or Ma'mum. Furthermore, what the Imam did was right, because when the Prophet (peace be upon him) forgot to sit for the first Tashahhud,
he offered Sujud-ul-Sahw (Prostration of Forgetfulness) according to the Hadith narrated on the authority of
`Abdullah ibn Buhaynah (may Allah be pleased with him).
(Part No. 7; Page No. 9) May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.