كتاب فضائل القرآن
Virtues of the Qur'an
Chapter: The Qurra from among the Companions of the Prophet (saws)
باب الْقُرَّاءِ مِنْ أَصْحَابِ النَّبِيِّ صلى الله عليه وسلم
Sahih al-Bukhari 5002

Narrated `Abdullah (bin Mas`ud):

By Allah other than Whom none has the right to be worshipped! There is no Sura revealed in Allah's Book but I know at what place it was revealed; and there is no Verse revealed in Allah's Book but I know about whom it was revealed. And if I know that there is somebody who knows Allah's Book better than I, and he is at a place that camels can reach, I would go to him.

حَدَّثَنَا عُمَرُ بْنُ حَفْصٍ، حَدَّثَنَا أَبِي، حَدَّثَنَا الأَعْمَشُ، حَدَّثَنَا مُسْلِمٌ، عَنْ مَسْرُوقٍ، قَالَ قَالَ عَبْدُ اللَّهِ ـ رضى الله عنه ـ وَاللَّهِ الَّذِي لاَ إِلَهَ غَيْرُهُ مَا أُنْزِلَتْ سُورَةٌ مِنْ كِتَابِ اللَّهِ إِلاَّ أَنَا أَعْلَمُ أَيْنَ أُنْزِلَتْ وَلاَ أُنْزِلَتْ آيَةٌ مِنْ كِتَابِ اللَّهِ إِلاَّ أَنَا أَعْلَمُ فِيمَ أُنْزِلَتْ، وَلَوْ أَعْلَمُ أَحَدًا أَعْلَمَ مِنِّي بِكِتَابِ اللَّهِ تُبَلِّغُهُ الإِبِلُ لَرَكِبْتُ إِلَيْهِ‏.‏
Reference : Sahih al-Bukhari 5002
In-book reference : Book 66, Hadith 24
USC-MSA web (English) reference : Vol. 6, Book 61, Hadith 524
  (deprecated numbering scheme)