Sahih Muslim Book 004, Hadith Number 1794.
Chapter : "The Qur'an has been revealed in seven modes of "reading" and its meaning.
Abu Wa'il reported: One day we went to 'Abdullah b. Mas'ud after we had observed the dawn prayer and we paid salutation at the door. He permitted us to enter, but we stayed for a while at the door, when the slave-girl came out and said: Why don't you come in? So we went in and (we found 'Abdullah b. Mas'ud) sitting and glorifying Allah (i.e. he was busy in dhikr) and he said: What obstructed you from coming in though you had been granted permission for it? We said: There was nothing (behind it) but we entertained the idea that some inmate of the house might be sleeping. He said: Do you presume any idleness on the part of the family of Ibn Umm 'Abd (the mother of Abdullah b. Mas'ud)? He was again busy with the glorification of Allah till he thought that the sun had risen. He said: Girl, see whether (the sun) has arisen. She glanced but it had not risen (by that time). He was again busy with the glorification (of Allah) and he (again) thought that the sun had arisen. She glanced (and confirmed) that, it had risen. Upon this he ('Abdullah b. Mas'ud) said: Praise be to Allah Who did not call us to account for our sins today. Mahdi said: I think that he said, He did not destroy us for our sins. One among the people said: I recited all the mufassal surahs during the night. 'Abdullah said: (You must have recited them) like the (recitation) of poetry. I heard (the Holy Prophet) combining (the sarahs) and I remember the combinations which the Messenger of Allah (may peace be upon him) made In the recitation (of surahs). These were constituted of eighteen mufassal surahs and two surahs (commencing with) Ha-Mim.