Imam al-Bukhari mentioned the following narration in his Saheeh, in the section on the merits of the Qur’an:
حَدَّثَنَا سَعِيدُ بْنُ عُفَيْرٍ، قَالَ حَدَّثَنِي اللَّيْثُ، قَالَ حَدَّثَنِي عُقَيْلٌ، عَنِ ابْنِ شِهَابٍ، قَالَ حَدَّثَنِي عُرْوَةُ بْنُ الزُّبَيْرِ، أَنَّ الْمِسْوَرَ بْنَ مَخْرَمَةَ، وَعَبْدَ الرَّحْمَنِ بْنَ عَبْدٍ الْقَارِيَّ، حَدَّثَاهُ أَنَّهُمَا، سَمِعَا عُمَرَ بْنَ الْخَطَّابِ، يَقُولُ سَمِعْتُ هِشَامَ بْنَ حَكِيمٍ، يَقْرَأُ سُورَةَ الْفُرْقَانِ فِي حَيَاةِ رَسُولِ اللَّهِ صلى الله عليه وسلم فَاسْتَمَعْتُ لِقِرَاءَتِهِ فَإِذَا هُوَ يَقْرَأُ عَلَى حُرُوفٍ كَثِيرَةٍ لَمْ يُقْرِئْنِيهَا رَسُولُ اللَّهِ صلى الله عليه وسلم فَكِدْتُ أُسَاوِرُهُ فِي الصَّلاَةِ فَتَصَبَّرْتُ حَتَّى سَلَّمَ فَلَبَّبْتُهُ بِرِدَائِهِ فَقُلْتُ مَنْ أَقْرَأَكَ هَذِهِ السُّورَةَ الَّتِي سَمِعْتُكَ تَقْرَأُ. قَالَ أَقْرَأَنِيهَا رَسُولُ اللَّهِ صلى الله عليه وسلم. فَقُلْتُ كَذَبْتَ فَإِنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَدْ أَقْرَأَنِيهَا عَلَى غَيْرِ مَا قَرَأْتَ، فَانْطَلَقْتُ بِهِ أَقُودُهُ إِلَى رَسُولِ اللَّهِ صلى الله عليه وسلم فَقُلْتُ إِنِّي سَمِعْتُ هَذَا يَقْرَأُ بِسُورَةِ الْفُرْقَانِ عَلَى حُرُوفٍ لَمْ تُقْرِئْنِيهَا. فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ” أَرْسِلْهُ اقْرَأْ يَا هِشَامُ ”. فَقَرَأَ عَلَيْهِ الْقِرَاءَةَ الَّتِي سَمِعْتُهُ يَقْرَأُ. فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ” كَذَلِكَ أُنْزِلَتْ ”. ثُمَّ قَالَ ” اقْرَأْ يَا عُمَرُ ”. فَقَرَأْتُ الْقِرَاءَةَ الَّتِي أَقْرَأَنِي، فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ” كَذَلِكَ أُنْزِلَتْ، إِنَّ هَذَا الْقُرْآنَ أُنْزِلَ عَلَى سَبْعَةِ أَحْرُفٍ فَاقْرَءُوا مَا تَيَسَّرَ مِنْهُ ”.
… al-Miswar ibn Makhramah and ‘Abd al-Rahman ibn ‘Abd al-Qari told us that they both heard ‘Umar ibn al-Khattab say:
I heard Hisham ibn Hakeem reciting surah al-Furqan while the Prophet was still alive, so I listened to his recitation and I noticed that he recited it differently in several ways from how Allah’s Messenger had recited it to me. I nearly struck him while he was still praying, but I held myself back until he completed the prayer and then I grabbed him by his shawl and said, “Who taught you this surah that I just heard you recite?” He said, “Allah’s Messenger recited it to me.” I said, “You have misspoken, for Allah’s Messenger recited it to me in a different way than how you recited it.”
So I immediately set off to see Allah’s Messenger, taking him along with me. I said to the Prophet, “I heard this person recite surah al-Furqan in some ways other than how you taught it to me.”
Allah’s Messenger said, “Let him go. Recite it, Hisham.” So he recited it in the way I had heard him recite it before. Then Allah’s Messenger, “That is how it was sent down,” and then said, “Recite it, ‘Umar.” So I recited it in the way I had been taught, and Allah’s Messenger said, “That is how it was sent down. This Qur’an was sent down in seven ahruf [dialects], so recite whatever of it is easy for you.”
[Saheeh al-Bukhari #4992]
While there has been much differing among the scholars as to the meaning of the seven ahruf, the correct meaning – inshaAllaah – is that they are different dialects of the Arabic language in which the Qur’an was revealed. However, as scholars have pointed out, ‘Umar and Hisham were both from the tribe of Quraysh and thus shared the same dialect. This raises the question: if they shared the same dialect, why would they differ in which harf they recited the Qur’an in?
The great scholar of hadith ibn Hajr al-‘Asqalaani addressed this point in his explanation of Saheeh al-Bukhari by writing:
قوله : ( فإن رسول الله – صلى الله عليه وسلم – قد أقرأنيها ) هذا قاله عمر استدلالا على ما ذهب إليه من تخطئة هشام ، وإنما ساغ له ذلك لرسوخ قدمه في الإسلام وسابقته ، بخلاف هشام فإنه كان قريب العهد بالإسلام فخشي عمر من ذلك أن لا يكون أتقن القراءة ، بخلاف نفسه فإنه كان قد أتقن ما سمع .ـ
Regarding ‘Umar’s statement, “For Allah’s Messenger recited it to me…”
‘Umar said this as an evidence for why he thought that Hisham was in error, and only reason why he thought this was because of his long-standing presence in Islam and his early acceptance of it, which was in contrast to Hisham’s condition, as he was still new to Islam. Because of that, ‘Umar feared that Hisham had not perfected his recitation, while he on the other hand ‘Umar had completely perfected what he had learned.
وكان سبب اختلاف قراءتهما أن عمر حفظ هذه السورة من رسول الله – صلى الله عليه وسلم – قديما ثم لم يسمع ما نزل فيها بخلاف ما حفظه وشاهده ، ولأن هشاما من مسلمة الفتح فكان النبي – صلى الله عليه وسلم – أقرأه على ما نزل أخيرا فنشأ [ ص: 643 ] اختلافهما من ذلك . ـ
So the reason for the difference between their recitations was that ‘Umar had learned this surah from Allah’s Messenger at a much earlier stage and had not subsequently heard what had been revealed [in the other ahruf – dialects – of the Qur’an] which differed from what he had memorized and heard directly, while Hisham was one of those who became at Muslim at the time of the Conquest of Mecca, and so the Prophet had recited this surah to him according to how it had been revealed later. So that was the source of the differences between them.
ومبادرة عمر للإنكار محمولة على أنه لم يكن سمع حديث : أنزل القرآن على سبعة أحرف إلا في هذه الوقعة . ـ
And ‘Umar’s quickness to disapprove of Hisham’s recitation could be interpreted to show that ‘Umar had not heard the hadith, “The Qur’an was sent down in seven ahruf” until this very event.
[Fath al-Bari #4706]
Read more of ibn Hajr’s explanation of this hadith here: Clarification on the Ahruf and Qiraa’aat of the Qur’an: ibn Hajr