What is the ruling on a congregant raising his voice with the "takbirat" and saying "Rabbana wa laka al-hamd" during congregational prayer?

It is better for the ma'mum (follower) not to vocalize behind the imam, whether in saying takbir for bowing or prostrating, or in saying "Rabbana wa lakal-hamd" (Our Lord, to You be all praise). If the imam's voice is audible, then repeating his takbir is not legislated. The prayer is not invalidated by the aforementioned vocalization, but those who do so should be advised and informed that it is not legislated.