Sahih Muslim Book 004, Hadith Number 1796.
Chapter : "The Qur'an has been revealed in seven modes of "reading" and its meaning.
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 (may peace be upon him) used to combine them, and he then mentioned twenty of the mufassal surahs, and (their combinations in) two in every rak'ah.