Sahih Bukhari Volume 009, Book 093, Hadith Number 640.

Narrated By 'Umar bin Al-Khattab : I heard Hisham bin Hakim reciting Surat-al-Furqan during the lifetime of Allah's Apostle, I listened to his recitation and noticed that he was reciting in a way that Allah's Apostle had not taught me. I was about to jump over him while He was still in prayer, but I waited patiently and when he finished his prayer, I put my sheet round his neck (and pulled him) and said, "Who has taught you this Sura which I have heard you reciting?" Hisham said, "Allah's Apostle taught it to me." I said, "You are telling a lie, for he taught it to me in a way different from the way you have recited it!" Then I started leading (dragged) him to Allah's Apostle and said (to the Prophet), " I have heard this man reciting Surat-al-Furqan in a way that you have not taught me." The Prophet said: "(O 'Umar) release him! Recite, O Hisham." Hisham recited in the way I heard him reciting. Allah's Apostle said, "It was revealed like this." Then Allah's Apostle said, "Recite, O 'Umar!" I recited in the way he had taught me, whereupon he said, "It was revealed like this," and added, "The Quran has been revealed to be recited in seven different ways, so recite of it whichever is easy for you." (See Hadith No. 514, Vol. 6)

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