Praise be to Allaah  

It is obligatory, as the Prophet (peace and blessings of Allaah be upon him) said: “pray as you have seen me praying.” Narrated by al-Bukhaari. And he (peace and blessings of Allaah be upon him) said: “There is no prayer for the one who does not recite the Opening of the Book al-Faatihah.” Saheeh – agreed upon. 

Kitaab Majmoo’ Fataawa wa Maqaalaat Mutanawwi’ah li Samaahat al-Shaykh ‘Abd al-‘Azeez ibn Baaz, 13/143.