What is incumbent upon a congregant who pronounced the opening takbir before the imam and continued with him until the end of the prayer?

Whoever says the opening takbir before his Imam, his prayer is invalid and he must repeat it. This is the view of the majority of scholars, whether he did so intentionally or out of forgetfulness. Their evidence is the Prophet's (peace be upon him) saying: "Indeed, the Imam is there to be followed, so do not differ from him. When he says the takbir, then say the takbir."