What is the ruling of repeating the takbir (Allahu akbar) after the imam during a congregational prayer? Is it still sunnah like when praying alone? What about the first takbir (iftitah)? I found out that my father doesn't repeat the takbirs after the imam (even the iftitah takbir!!!) and he told me that he followed the imam so he didn't need to repeat. Is this logic correct? Are his congregational prayers without saying the iftitah takbir valid?