Inni wajjahtu wajhiya lillahi fatiri s-mawaati wal ard hanii-fa-n wamaa ana minal mushrikeen.
For example, let's say I want to pray Maghrib (3 raka'a fard and 2 raka'a sunnah). So, I first recite this dua and pray 3 raka'a fard. But before praying the 2 raka'a sunnah, do I have to recite this dua again? Does this dua go before every salah?