Abu Wa'il narrates: A man came to Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) and said: O Abu Abd al-Rahman, should I recite behind the imam? He said: "Listen to the Qur'an in silence, for in the prayer there is an occupation (of its own), and the imam will suffice you for that."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9279