(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).
Reference: Sahih Muslim 1091 a
In-book reference: Book 13, Hadith 40
USC-MSA web (English) reference(deprecated numbering scheme): Book 6, Hadith 2397