Language:
كتاب فضائل القرآن
66  
Virtues of the Qur'an
(5)
Chapter: The Qur'an was revealed to be recited in seven different ways
(5)
باب أُنْزِلَ الْقُرْآنُ عَلَى سَبْعَةِ أَحْرُفٍ
Sahih al-Bukhari 4991

Narrated `Abdullah bin `Abbas:

Allah's Messenger (ﷺ) said, "Gabriel recited the Qur'an to me in one way. Then I requested him (to read it in another way), and continued asking him to recite it in other ways, and he recited it in several ways till he ultimately recited it in seven different ways."

حَدَّثَنَا سَعِيدُ بْنُ عُفَيْرٍ، قَالَ حَدَّثَنِي اللَّيْثُ، قَالَ حَدَّثَنِي عُقَيْلٌ، عَنِ ابْنِ شِهَابٍ، قَالَ حَدَّثَنِي عُبَيْدُ اللَّهِ بْنُ عَبْدِ اللَّهِ، أَنَّ ابْنَ عَبَّاسٍ ـ رضى الله عنهما ـ حَدَّثَهُ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ أَقْرَأَنِي جِبْرِيلُ عَلَى حَرْفٍ فَرَاجَعْتُهُ، فَلَمْ أَزَلْ أَسْتَزِيدُهُ وَيَزِيدُنِي حَتَّى انْتَهَى إِلَى سَبْعَةِ أَحْرُفٍ ‏"‏‏.‏
Reference : Sahih al-Bukhari 4991
In-book reference : Book 66, Hadith 13
USC-MSA web (English) reference : Vol. 6, Book 61, Hadith 513
  (deprecated numbering scheme)