Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.142.

The Prophet, upon whom be peace, did not confine his recitation of the Qur’an in prayers to some specific surahs, (except for the Friday and ‘Id prayers). Concerning the other prayers, Abu Dawud has recorded a hadith from ‘Amr ibn Shu’aib from his father on the authority of his grandfather who said, “There is no separate surah, large or small, except the ones I heard the Prophet recite while leading the people in one of the obligatory prayers. He used to recite the entire surah in two rak’ahs, or just the initial part of the surah. It has not been recorded from him that he would recite from the middle or the end of the surah, nor that he would recite two surahs in one rak’ah during the obligatory prayers. He would, however, do so during voluntary prayers. Said Ibn Mas’ud, “I know the surahs the Prophet used to recite together in one rak’ah: ar-Rahman and an-Najm, al-Qamar and al-Haqqah, at-Tur and azh-Zhariyat, al-Waqi’ah and Noon, and so on.” But this hadith does not tell us if this was during obligatory or voluntary prayers. The latter is more probable. He rarely recited one surah in two (both) rak’ahs. Abu Dawud records that a man from the tribe of Juhainah heard the Prophet, upon whom be peace, recite the complete surah az-Zil~al twice in both rak’ahs of the morning prayer. The man commented, “I do not know if he did this out of forgetfulness or if he recited it twice intentionally.”