From Sahih Muslim (Hadith 1913):
Abu Wa'il reported:A person came to 'Abdullah b. Mas'ud and said: I recited all the mufassal surahs in one rak'ah during the night. 'Abdullah said: You must have recited hastily like the recitation of poetry. 'Abdullah said: I remember well the manner in which the Messenger of Allah (ﷺ) used to combine them, and he then mentioned twenty of the mufassal surahs, and (their combinations in) two in every rak'ah
Reference: Muslim 1913