'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.
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، قَالَ قَرَأْتُ عَلَى مَالِكٍ عَنِ ابْنِ شِهَابٍ، عَنْ عُرْوَةَ بْنِ الزُّبَيْرِ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ عَبْدٍ الْقَارِيِّ، قَالَ سَمِعْتُ عُمَرَ بْنَ الْخَطَّابِ، يَقُولُ سَمِعْتُ هِشَامَ بْنَ، حَكِيمِ بْنِ حِزَامٍ يَقْرَأُ سُورَةَ الْفُرْقَانِ عَلَى غَيْرِ مَا أَقْرَؤُهَا وَكَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم أَقْرَأَنِيهَا فَكِدْتُ أَنْ أَعْجَلَ عَلَيْهِ ثُمَّ أَمْهَلْتُهُ حَتَّى انْصَرَفَ ثُمَّ لَبَّبْتُهُ بِرِدَائِهِ فَجِئْتُ بِهِ رَسُولَ اللَّهِ صلى الله عليه وسلم فَقُلْتُ يَا رَسُولَ اللَّهِ إِنِّي سَمِعْتُ هَذَا يَقْرَأُ سُورَةَ الْفُرْقَانِ عَلَى غَيْرِ مَا أَقْرَأْتَنِيهَا ‏.‏ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ أَرْسِلْهُ اقْرَأْ ‏"‏ ‏.‏ فَقَرَأَ الْقِرَاءَةَ الَّتِي سَمِعْتُهُ يَقْرَأُ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ هَكَذَا أُنْزِلَتْ ‏"‏ ‏.‏ ثُمَّ قَالَ لِيَ ‏"‏ اقْرَأْ ‏"‏ ‏.‏ فَقَرَأْتُ فَقَالَ ‏"‏ هَكَذَا أُنْزِلَتْ إِنَّ هَذَا الْقُرْآنَ أُنْزِلَ عَلَى سَبْعَةِ أَحْرُفٍ فَاقْرَءُوا مَا تَيَسَّرَ مِنْهُ ‏"‏ ‏.‏
Reference : Sahih Muslim 818 a
In-book reference : Book 6, Hadith 327
USC-MSA web (English) reference : Book 4, Hadith 1782
  (deprecated numbering scheme)