Sahih Muslim 822 b
Abu Wa'il reported:
A person came to 'Abdullah, who was called Nahik b. Sinan, and the rest of the hadith is the same but for this:" Alqama came to him ('Abdullah b. Mas'ud) and we said to him: Ask him about the manners in which he combined (two surahs) in one rak'ah. So he went to him and asked him and then came to us and said: Twenty are the mufassal surahs in the compilation (of the Qur'an) made by 'Abdullah."
وَحَدَّثَنَا أَبُو كُرَيْبٍ، حَدَّثَنَا أَبُو مُعَاوِيَةَ، عَنِ الأَعْمَشِ، عَنْ أَبِي وَائِلٍ، قَالَ جَاءَ رَجُلٌ إِلَى عَبْدِ اللَّهِ يُقَالُ لَهُ نَهِيكُ بْنُ سِنَانٍ ‏.‏ بِمِثْلِ حَدِيثِ وَكِيعٍ غَيْرَ أَنَّهُ قَالَ فَجَاءَ عَلْقَمَةُ لِيَدْخُلَ عَلَيْهِ فَقُلْنَا لَهُ سَلْهُ عَنِ النَّظَائِرِ الَّتِي كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يَقْرَأُ بِهَا فِي رَكْعَةٍ فَدَخَلَ عَلَيْهِ فَسَأَلَهُ ثُمَّ خَرَجَ عَلَيْنَا فَقَالَ عِشْرُونَ سُورَةً مِنَ الْمُفَصَّلِ فِي تَأْلِيفِ عَبْدِ اللَّهِ ‏.‏