An Imam forgetting the first Tashahhud

A: The Salah of all of them is correct, because each of them based it on what they thought was most probably correct. It would have been better for those who separated from the Iman not to have done so and to have stayed with their Imam, because his return to the sitting position is permissible, as he had stood up but had not started to recite (Surah Al-Fatihah). Even if that he had returned to the sitting position after starting the recitation, due to ignorance of the legal ruling, the Salah would still have been valid.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.