Musnad Ahmad 26:360

'Affan narrated to us, Hammad narrated to us, 'Asim narrated to us, from Zirr, that a man said to Ibn Mas'ud: "How do you read this letter: *ma'in ghayri yasin* (water that does not change) or *asin* (stagnant/putrid)?" He said: "Have you read the entire Qur'an?" The man said: "I indeed recite the *Mufassal* (the chapters from Qaf to the end of the Qur'an) in its entirety in a single rak'ah." Ibn Mas'ud said: "Is it like poetry, may you have no father? I have learned the pairings of the Messenger of Allah, peace and blessings of Allah be upon him, which he used to pair—two chapters, two chapters—from the beginning of the *Mufassal*." And the beginning of the *Mufassal* for Ibn Mas'ud was: (Surah) *Ar-Rahman*.