Musnad Ahmad 1012:6

Muhammad ibn Salamah narrated to us, from Ibn Ishaq, 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 prayer and recited, but the recitation became difficult for him. When he finished, he said: "Do you recite [behind the Imam]?" We said: "Yes, O Messenger of Allah." He said: "Do not do so, except for the Opening of the Book (Fatihat al-Kitab), for there is no prayer for the one who does not recite it."