Muwatta Imam Malik Book of Prayer, Hadith 45

Yahya related to me from Malik from Nafi that Abdullah ibn Umar, when asked if anyone should recite behind an imam, said, "When you pray behind an imam then the recitation of the imam is enough for you and when you pray on your own you must recite." Nafi added, "Abdullah ibn 'Umar used not to recite behind the imam."
Yahya said that he heard Malik say, "The position with us is that aman recites behind the imam when the imam does not recite aloud and he refrains from reciting when the imam recites aloud."

حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ إِذَا سُئِلَ هَلْ يَقْرَأُ أَحَدٌ خَلْفَ الإِمَامِ قَالَ إِذَا صَلَّى أَحَدُكُمْ خَلْفَ الإِمَامِ فَحَسْبُهُ قِرَاءَةُ الإِمَامِ وَإِذَا صَلَّى وَحْدَهُ فَلْيَقْرَأْ . قَالَ وَكَانَ عَبْدُ اللَّهِ بْنُ عُمَرَ لاَ يَقْرَأُ خَلْفَ الإِمَامِ . قَالَ يَحْيَى سَمِعْتُ مَالِكًا يَقُولُ الأَمْرُ عِنْدَنَا أَنْ يَقْرَأَ الرَّجُلُ وَرَاءَ الإِمَامِ فِيمَا لاَ يَجْهَرُ فِيهِ الإِمَامُ بِالْقِرَاءَةِ وَيَتْرُكُ الْقِرَاءَةَ فِيمَا يَجْهَرُ فِيهِ الإِمَامُ بِالْقِرَاءَةِ .