Hadith 10:170
Narrated Abu Wa'il:
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 (ﷺ) 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) Ha, Meem [??] (which the Prophet (ﷺ) used to recite) in each rak`a.
حَدَّثَنَا آدَمُ، قَالَ حَدَّثَنَا شُعْبَةُ، عَنْ عَمْرِو بْنِ مُرَّةَ، قَالَ سَمِعْتُ أَبَا وَائِلٍ، قَالَ جَاءَ رَجُلٌ إِلَى ابْنِ مَسْعُودٍ فَقَالَ قَرَأْتُ الْمُفَصَّلَ اللَّيْلَةَ فِي رَكْعَةٍ. فَقَالَ هَذًّا كَهَذِّ الشِّعْرِ لَقَدْ عَرَفْتُ النَّظَائِرَ الَّتِي كَانَ النَّبِيُّ صلى الله عليه وسلم يَقْرِنُ بَيْنَهُنَّ فَذَكَرَ عِشْرِينَ سُورَةً مِنَ الْمُفَصَّلِ سُورَتَيْنِ فِي كُلِّ رَكْعَةٍ
Original Reference: Sahih al-Bukhari 775