ANSWER:

You must say both when praying alone. When praying behind an imam, you only say rabbana wa lakal hamd. 

You should say Rabbana wa lakal hamd while rising up after the imam finishes his statement. 

While standing after ruku, i say “sami-allah hulimaan hamidah” then i say “rabbana wa lakal hamd” but i’ve found many of the people saying only “rabbana lakal hamd” after ruku. I wanted to know if i’m doing something wrong or bidaah by adding “sami-Allah hulimaan hamidah”? Or i can say this? And when praying behind the imam should we say it while we are moving from Ruku to standing position or after standing straight completely?