كتاب الصيام
13
The Book of Fasting
(8)
Chapter: Clarifying that fasting begins at dawn, and a person may eat and other than that until dawn begins; And clarifying the dawn which has to do with the rulings concerning the beginning of fasting and the beginning of the time for the Subh Prayer, and other than that, which is the Second Dawn, which is called the True Dawn. The First Dawn, which is the False Dawn, has nothing to do with the rulings
(8)
بَاب بَيَانِ أَنَّ الدُّخُولَ فِي الصَّوْمِ يَحْصُلُ بِطُلُوعِ الْفَجْرِ وَأَنَّ لَهُ الْأَكْلَ وَغَيْرَهُ حَتَّى يَطْلُعَ الْفَجْرُ وَبَيَانِ صِفَةِ الْفَجْرِ الَّذِي تَتَعَلَّقُ بِهِ الْأَحْكَامُ مِنْ الدُّخُولِ فِي الصَّوْمِ وَدُخُولِ وَقْتِ صَلَاةِ الصُّبْحِ وَغَيْرِ ذَلِكَ

Sahl b. Sa'd said that when this verse was revealed:

" Eat and drink till the white streak is distinct from the dark streak," a person would take hold of a white thread and a black thread and keep on eating till he could find them distinct (in the light of the dawn). It was then that Allah, the Majestic and Great, reveiled (the words) min al-fajr (from the dawn), and then it became clear (that the word khait refers to the streak of light in the dawn).
حَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ عُمَرَ الْقَوَارِيرِيُّ، حَدَّثَنَا فُضَيْلُ بْنُ سُلَيْمَانَ، حَدَّثَنَا أَبُو حَازِمٍ، حَدَّثَنَا سَهْلُ بْنُ سَعْدٍ، قَالَ لَمَّا نَزَلَتْ هَذِهِ الآيَةُ ‏{‏ وَكُلُوا وَاشْرَبُوا حَتَّى يَتَبَيَّنَ لَكُمُ الْخَيْطُ الأَبْيَضُ مِنَ الْخَيْطِ الأَسْوَدِ‏}‏ قَالَ كَانَ الرَّجُلُ يَأْخُذُ خَيْطًا أَبْيَضَ وَخَيْطًا أَسْوَدَ فَيَأْكُلُ حَتَّى يَسْتَبِينَهُمَا حَتَّى أَنْزَلَ اللَّهُ عَزَّ وَجَلَّ ‏{‏ مِنَ الْفَجْرِ‏}‏ فَبَيَّنَ ذَلِكَ ‏.‏
Reference : Sahih Muslim 1091 a
In-book reference : Book 13, Hadith 40
USC-MSA web (English) reference : Book 6, Hadith 2397
  (deprecated numbering scheme)