(a) It is narrated from Sayyiduna Thabit (may Allah have mercy on him) that Sayyiduna Anas (may Allah be pleased with him) used to order us to recite even behind the imam. Thabit (may Allah have mercy on him) says: I used to stand beside Sayyiduna Anas (may Allah be pleased with him), and he would recite Surah al-Fatihah and one surah from the Mufassal, and he would let us hear his recitation so that we could learn it from him.
(b) Ibn Khuzaymah (may Allah have mercy on him) narrated this hadith in 'al-Qira'ah Khalf al-Imam' (Recitation Behind the Imam) from Ahmad bin Sa'id al-Darimi (may Allah have mercy on him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2947