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