Musnad Ahmad 1012:29

Yazid narrated to us, he said: Muhammad ibn Ishaq informed us, from Makhul, from Mahmud ibn al-Rabi’, from Ubadah ibn al-Samit, who said: The Messenger of Allah (peace and blessings of Allah be upon him) led us in the morning prayer, and the recitation became difficult for him. When he finished, he said: "I see you reciting behind your Imam." We said: "Yes, by Allah, O Messenger of Allah, we do indeed do that." He said: "Do not do so, except for the Mother of the Quran (Al-Fatiha), for there is no prayer for the one who does not recite it."