Sahih Bukhari - Hadith No: 561

I heard Hisham bin Hakim bin Hizam reciting Surat-al-Furqan during the lifetime of Allah's Apostle, and I listened to his recitation and noticed that he recited it in several ways which Allah's Apostle had not taught me. So I was on the point of attacking him in the prayer, but I waited till he finished his prayer, and then I seized him by the collar and said, "Who taught you this Surah which I have heard you reciting?" He replied, "Allah's Apostle taught it to me." I said, "You are telling a lie; By Allah! Allah's Apostle taught me (in a different way) this very Surah which I have heard you reciting." So I took him, leading him to Allah's Apostle and said, "O Allah's Apostle! I heard this person reciting Surat-al-Furqan in a way that you did not teach me, and you have taught me Surat-al-Furqan." The Prophet said, "O Hisham, recite!" So he recited in the same way as I heard him recite it before. On that Allah's Apostle said, "It was revealed to be recited in this way." Then Allah's Apostle said, "Recite, O 'Umar!" So I recited it as he had taught me. Allah's Apostle then said, "It was revealed to be recited in this way." Allah" Apostle added, "The Quran has been revealed to be recited in several different ways, so recite of it that which is easier for you."