Sahih Bukhari, Chapter: Characteristics of Prayer

A man came to Ibn Mas'ud and said, "I recited the Mufassal (Suras) at night in one Rak'a." Ibn Mas'ud said, "This recitation is (too quick) like the recitation of poetry. I know the identical Suras which the Prophet used to recite in pairs." Ibn Mas'ud then mentioned 20 Mufassal Suras including two Suras from the family of (i.e. those verses which begin with) AL, HA, MIM (which the Prophet used to recite) in each Rak'a.