Sunnah method of Salaah
Q: When you perform salaah, when doing takbeer to start, should we look up or down? And after raising from rukuand saying "samirallahhulimanhamida", some says you have to say "allahumma rabbanawakalhamd" and others only "rabbana lakalhamd"? Which one is correct according to sunnah? And is it obligatory to say it? If i did not say it, are all my prayers invalid?