Sahih Bukhari Volume 6, Book 65, Hadith 4513

حَدَّثَنَا ابْنُ أَبِي مَرْيَمَ، حَدَّثَنَا أَبُو غَسَّانَ، مُحَمَّدُ بْنُ مُطَرِّفٍ حَدَّثَنِي أَبُو حَازِمٍ، عَنْ سَهْلِ بْنِ سَعْدٍ، قَالَ وَأُنْزِلَتْ ‏{‏وَكُلُوا وَاشْرَبُوا حَتَّى يَتَبَيَّنَ لَكُمُ الْخَيْطُ الأَبْيَضُ مِنَ الْخَيْطِ الأَسْوَدِ‏}‏ وَلَمْ يُنْزَلْ ‏{‏مِنَ الْفَجْرِ‏}‏ وَكَانَ رِجَالٌ إِذَا أَرَادُوا الصَّوْمَ رَبَطَ أَحَدُهُمْ فِي رِجْلَيْهِ الْخَيْطَ الأَبْيَضَ وَالْخَيْطَ الأَسْوَدَ، وَلاَ يَزَالُ يَأْكُلُ حَتَّى يَتَبَيَّنَ لَهُ رُؤْيَتُهُمَا، فَأَنْزَلَ اللَّهُ بَعْدَهُ ‏{‏مِنَ الْفَجْرِ‏}‏ فَعَلِمُوا أَنَّمَا يَعْنِي اللَّيْلَ مِنَ النَّهَارِ‏.‏

Narrated By Sahl bin Sad : The Verse: "And eat and drink until the white thread appears to you distinct from the black thread." was revealed, but: '...of dawn' was not revealed (along with it) so some men, when intending to fast, used to tie their legs, one with white thread and the other with black thread and would keep on eating till they could distinguish one thread from the other. Then Allah revealed'... of dawn,' whereupon they understood that meant the night and the day.