Sahih Bukhari Volume 006, Book 061, Hadith Number 518.
Narrated By Shaqiq : Abdullah said, "I learnt An-Naza'ir which the Prophet used to recite in pairs in each Rak'a." Then Abdullah got up and Alqama accompanied him to his house, and when Alqama came out, we asked him (about those Suras). He said, "They are twenty Suras that start from the beginning of Al-Mufassal, according to the arrangement done be Ibn Mas'ud, and end with the Suras starting with Ha Mim, e.g. Ha Mim (the Smoke). and "About what they question one another?" (78.1)