Sahih Muslim Book 004, Hadith Number 1791.
Chapter : "The Qur'an has been revealed in seven modes of "reading" and its meaning.
Abu Wa'il reported that a person named Nabik b. Sinan came to Abdullah (b. Mas'ud) and said: Abu 'Abd al-Rahman, how do you recite this word (alif) or (ya)? Would you read It as: min ma'in ghaira asin or au min ma'in ghaira ghaira yasin. (al-Qur'an, xlvii. 15)? 'Abdullah said: You (seem to) have memorised the whole of the Qur'an except this. He (again) said: I recite all the mufassal surahs in one rak'ah. Upon this 'Abdullah said: (You must have been reciting It) hastily like the recitation of poetry. Verily. there are people who recite the Qur'an, but it does not go down beyond their collar bones. It is (a fact with the Qur'an) that it is beneficial only when it settles in the heart and is rooted deeply in it. The best of (the acts) in prayer are bowing and prostration. I am quite aware of the occasions when the Messenger of Allah (may peace be upon him) combined together two surahs in every rak'ah. 'Abdullah then stood up and went out with 'Alqama following in his footstep. He said Ibn Numair had told him that the narration was like that: "A person belonging to Banu Bajila came to 'Abdullah," and he did not mention (the name of) Nahik b. Sinan.