(a) Shu'bah said: "I said to Qatadah: Perhaps he did not approve of reciting behind the imam?" He said: "Had he not approved of it, he would certainly have forbidden it."
(b) It has been narrated to us from Imran bin Husayn (may Allah be pleased with him) that he said: "No prayer is valid without Surah al-Fatihah." If the Prophet (peace and blessings be upon him) disapproved of the one reciting behind him, it was his reciting aloud that he disliked, not reciting within oneself (that is, he disapproved of his recitation because he was reciting aloud, which was disturbing his own recitation).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2907