It is narrated from Sayyiduna Ubadah bin Samit (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever does not recite Umm al-Qur'an (al-Fatihah), his prayer is not valid."
Hadith Referenceسنن دارمي / من كتاب الصللاة / 1276
The word of the hadith, «لَاصَلَاةَ», is indefinite (nakirah) and indicates generality. Its meaning is that whether the prayer is obligatory (fard) or voluntary (nafl), silent (sirri) or aloud (jahri), performed behind an imam or individually, if Surah al-Fatihah is not recited in it, then that prayer will not be valid. Therefore, Surah al-Fatihah must be recited in every rak‘ah of every prayer, whether one is praying alone or behind an imam; in every case, Surah al-Fatihah must be recited.