Sayyiduna Ubadah bin Samit (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) led us in the morning prayer, and recitation became heavy and difficult for him. Then, when he finished the prayer, he asked: "Surely, I think you recite behind your Imam?" They say, we replied, "Yes, by Allah, O Messenger of Allah (we recite quickly)." The Prophet (peace and blessings be upon him) said: "Then do not recite anything except Umm al-Kitab, because whoever does not recite Surah Al-Fatihah, his prayer is not valid." This hadith is from Ibn Ulayyah and Abdul A'la.