Sayyiduna Ubadah bin al-Samit (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Whoever does not recite Surah al-Fatihah in the prayer, his prayer is not valid."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2914