Hisham bin Urwah (may Allah have mercy on him) narrates from his father (may Allah have mercy on him) that he said: O my dear sons! Recite (al-Fatihah) during the imam's pause, for "There is no prayer without Surah al-Fatihah."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2954