Sahih al-Bukhari 1917
Sahih al-Bukhari · Book 30: Fasting · The Statement of Allah Taa'la: "And eat and drink until the white thread of dawn appears to you distinct from the black."
Narrated Sahl bin Saud:
حَدَّثَنَا سَعِيدُ بْنُ أَبِي مَرْيَمَ ، حَدَّثَنَا ابْنُ أَبِي حَازِمٍ ، عَنْ أَبِيهِ ، عَنْ سَهْلِ بْنِ سَعْدٍ ، ح. حَدَّثَنِي سَعِيدُ بْنُ أَبِي مَرْيَمَ ، حَدَّثَنَا أَبُو غَسَّانَ، مُحَمَّدُ بْنُ مُطَرِّفٍ قَالَ حَدَّثَنِي أَبُو حَازِمٍ ، عَنْ سَهْلِ بْنِ سَعْدٍ ، قَالَ أُنْزِلَتْ {وَكُلُوا وَاشْرَبُوا حَتَّى يَتَبَيَّنَ لَكُمُ الْخَيْطُ الأَبْيَضُ مِنَ الْخَيْطِ الأَسْوَدِ} وَلَمْ يَنْزِلْ مِنَ الْفَجْرِ، فَكَانَ رِجَالٌ إِذَا أَرَادُوا الصَّوْمَ رَبَطَ أَحَدُهُمْ فِي رِجْلِهِ الْخَيْطَ الأَبْيَضَ وَالْخَيْطَ الأَسْوَدَ، وَلَمْ يَزَلْ يَأْكُلُ حَتَّى يَتَبَيَّنَ لَهُ رُؤْيَتُهُمَا، فَأَنْزَلَ اللَّهُ بَعْدُ {مِنَ الْفَجْرِ} فَعَلِمُوا أَنَّهُ إِنَّمَا يَعْنِي اللَّيْلَ وَالنَّهَارَ.
When the following verses were revealed: 'Eat and drink until the white thread appears to you, distinct from the black thread' and of dawn was not revealed, some people who intended to fast, tied black and white threads to their legs and went on eating till they differentiated between the two. Allah then revealed the words, 'of dawn', and it became clear that meant night and day.