From Sahih Muslim (Hadith 1909):
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
Reference: Muslim 1909