Sahih Muslim 818 a
The Book of Prayer - Travellers
The clarification that the Qur’an was revealed in seven ahruf (modes of recitation), and clarifying its meaning
'Umar b. Khattab said: I heard Hisham b. Hakim b. Hizam reciting Surah al-Furqan in a style different from that in which I used to recite it, and in which Allah's Messenger (ﷺ) had taught me to recite it. I was about to dispute with him (on this style) but I delayed till he had finished that (the recitation). Then I caught hold of his cloak and brought him to the Messenger of Allah (ﷺ) and said: Messenger of Allah, I heard this man reciting Surah al-Furqan in a style different from the one in which you taught me to recite. Upon this the Messenger of Allah (ﷺ) told (me) to leave him alone and asked him to recite. He then recited in the style in which I beard him recite it. The Messenger of Allah (ﷺ) then said: Thus was it sent down. He then told me to recite and I recited it, and he said: Thus was it sent down. The Qur'an was sent down in seven dialects. So recite what seems easy therefrom.