Language:
كتاب الأذان
10  
Call to Prayers (Adhaan)
(106)
Chapter: To recite two Surah in one Rak'a and to recite the last verses of some Surah, or to elite the Surah in their reverse order, or to recite the beginning of a Surah
(106)
باب الْجَمْعِ بَيْنَ السُّورَتَيْنِ فِي الرَّكْعَةِ
Sahih al-Bukhari 775

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.‏
Reference : Sahih al-Bukhari 775
In-book reference : Book 10, Hadith 170
USC-MSA web (English) reference : Vol. 1, Book 12, Hadith 742
  (deprecated numbering scheme)