Mishkat al-Masabih 2211
‘Umar b. al-Khattab said:
I heard Hishām b. Hakīm b. Hizām reciting sūra al-Furqān (Qur’ān, 25) in a different manner from my way of reciting it, and God’s messenger had taught me to recite it. I nearly spoke sharply to him, but I delayed till he had finished, and then catching his cloak at the neck I brought him to God’s messenger and said, “Messen­ger of God, I heard this man reciting sūra al-Furqān in a manner different from that in which you taught me to recite it.” He told me to let him go and told him to recite. When he recited it in the manner in which I had heard him recite it God's messenger said, “Thus was it sent down.” He then told me to recite it and when I had done so he said, “Thus was it sent down. The Qur’ān was sent down in seven modes of reading, so recite according to what comes most easily.” 

(Bukhārī and Muslim, the wording being Muslim’s.)
وَعَنْ عُمَرَ بْنِ الْخَطَّابِ رَضِيَ اللَّهُ عَنْهُ قَالَ: سَمِعْتُ هِشَامَ بْنَ حَكِيمِ بْنِ حِزَامٍ يقْرَأ سُورَة الْفرْقَان على غير مَا أقرؤوها. وَكَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ أَقْرَأَنِيهَا فَكِدْتُ أَنْ أَعْجَلَ عَلَيْهِ ثُمَّ أَمْهَلْتُهُ حَتَّى انْصَرَفَ ثُمَّ لَبَّبْتُهُ بِرِدَائِهِ فَجِئْتُ بِهِ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ. فَقلت يَا رَسُولَ اللَّهِ إِنِّي سَمِعْتُ هَذَا يَقْرَأُ سُورَةَ الْفُرْقَانِ عَلَى غَيْرِ مَا أَقْرَأْتَنِيهَا. فَقَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: أَرْسِلْهُ اقْرَأ " فَقَرَأت الْقِرَاءَةَ الَّتِي سَمِعْتُهُ يَقْرَأُ. فَقَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «هَكَذَا أُنْزِلَتْ» . ثُمَّ قَالَ لي: «اقْرَأ» . فَقَرَأت. فَقَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «هَكَذَا أنزلت إِن الْقُرْآنَ أُنْزِلَ عَلَى سَبْعَةِ أَحْرُفٍ فَاقْرَءُوا مَا تيَسّر مِنْهُ» . مُتَّفق عَلَيْهِ. وَاللَّفْظ لمُسلم