Yahya related to me from Malik from Nafi that Abdullah ibn Umar,when asked if anyone should recite behind an imam, said, "When youpray behind an imam then the recitation of the imam is enough for youand when you pray on your own you must recite." Nafi added, "Abdullahibn 'Umar used not to recite behind the imam."Yahya saidthat he heard Malik say, "The position with us is that aman recitesbehind the imam when the imam does not recite aloud and he refrainsfrom reciting when the imam recites aloud."
App reference: Book 3, Hadith 45
USC-MSA web (English) reference: Book 3, Hadith 45
Arabic reference: Book 3, Hadith 192