Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1793
This hadith has been narrated by A'mash with the same chain of transmitters in which ('Abdullah b. Mas'ud) said:" I know the manners in which the Messenger of Allah (may peace be upon him) recited the two surahs in one rak'ah and then twenty surahs in ten rak'ahs."