It is narrated from Nafi' (may Allah have mercy on him) that Ibn Umar (may Allah be pleased with them both), when he prayed four rak'ahs alone, would recite Surah Al-Fatihah and a surah from the Qur'an in every rak'ah. Nafi' says, and sometimes in one rak'ah of an obligatory prayer, he would recite two or three surahs as well.
Hadith Referenceمسند الإمام الشافعي / كتاب الصلاة / 219
Hadith Takhrijاخرجه مالك في الموطا: 1 / 79، وعبد الرزاق: (2723)، كتاب الأم: 7 / 207