Mishkat al-Masabih 1199
Abdallah b. Mas'ud said, “I know the comparable suras which the Prophet used to join together; ” and he mentioned twenty suras at the beginning of al-Mufassal,1 according to Ibn Mas'ud’s arrangement, two in a rak'a, the last of them being H. M. ad-Dukhan,2 and “About what do they ask one another?”3 

    1. A title given to the suras from 49 to the end, but several other suras are also mentioned:
37, 45, 47, 48, 50, 61, 67, 87, and 93. The name is most appropriately explained ay meaning that this is the section of the Qur’an which contains many shorter suras.
    2. Al-Qur’an; 94. As this is included as one of the suras in al Mufassal , it suggests that this section begins earlier than the first number mentioned in the note referred to above.
    3. Al-Qur'an; 78.

(Bukhari and Muslim.)
وَعَنْ عَبْدِ اللَّهِ بْنِ مَسْعُودٍ قَالَ: لَقَدْ عَرَفْتُ النَّظَائِرَ الَّتِي كَانَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقْرِنُ بَيْنَهُنَّ فَذَكَرَ عِشْرِينَ سُورَةً مِنْ أَوَّلِ الْمُفَصَّلِ عَلَى تَأْلِيفِ ابْنِ مَسْعُودٍ سُورَتَيْنِ فِي رَكْعَةٍ آخِرُهُنَّ (حم الدُّخان)
و (عَم يتساءلون)