"Mandatory reading Al-Fatihah in the first and second rak'ah while in the third and fourth rak'ah is Sunnah"
get Hadiths:
From Ubadah bin Saamit radi 'anhu: he said: That the Prophet sallallaahu' alaihi wa sallam said: People who do not read Al-Fatihah, invalid prayers. (Narrated by Muslim)