Language:
كتاب الصوم
30  
Fasting
(16)
Chapter: The Statement of Allah Taa'la: "And eat and drink until the white thread of dawn appears to you distinct from the black."
(16)
بَابُ قَوْلِ اللَّهِ تَعَالَى: {وَكُلُوا وَاشْرَبُوا حَتَّى يَتَبَيَّنَ لَكُمُ الْخَيْطُ الأَبْيَضُ مِنَ الْخَيْطِ الأَسْوَدِ مِنَ الْفَجْرِ ثُمَّ أَتِمُّوا الصِّيَامَ إِلَى اللَّيْلِ}
Sahih al-Bukhari 1916

Narrated `Adi bin Hatim:

When the above verses were revealed: 'Until the white thread appears to you, distinct from the black thread,' I took two (hair) strings, one black and the other white, and kept them under my pillow and went on looking at them throughout the night but could not make anything out of it. So, the next morning I went to Allah's Messenger (ﷺ) and told him the whole story. He explained to me, "That verse means the darkness of the night and the whiteness of the dawn."

حَدَّثَنَا حَجَّاجُ بْنُ مِنْهَالٍ، حَدَّثَنَا هُشَيْمٌ، قَالَ أَخْبَرَنِي حُصَيْنُ بْنُ عَبْدِ الرَّحْمَنِ، عَنِ الشَّعْبِيِّ، عَنْ عَدِيِّ بْنِ حَاتِمٍ ـ رضى الله عنه ـ قَالَ لَمَّا نَزَلَتْ ‏{‏حَتَّى يَتَبَيَّنَ لَكُمُ الْخَيْطُ الأَبْيَضُ مِنَ الْخَيْطِ الأَسْوَدِ‏}‏ عَمَدْتُ إِلَى عِقَالٍ أَسْوَدَ وَإِلَى عِقَالٍ أَبْيَضَ، فَجَعَلْتُهُمَا تَحْتَ وِسَادَتِي، فَجَعَلْتُ أَنْظُرُ فِي اللَّيْلِ، فَلاَ يَسْتَبِينُ لِي، فَغَدَوْتُ عَلَى رَسُولِ اللَّهِ صلى الله عليه وسلم فَذَكَرْتُ لَهُ ذَلِكَ فَقَالَ ‏"‏ إِنَّمَا ذَلِكَ سَوَادُ اللَّيْلِ وَبَيَاضُ النَّهَارِ ‏"‏‏.‏
Reference : Sahih al-Bukhari 1916
In-book reference : Book 30, Hadith 26
USC-MSA web (English) reference : Vol. 3, Book 31, Hadith 140
  (deprecated numbering scheme)