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.  

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  

When this verse was revealed." Eat and drink till the white streak becomes distinct from the dark streak for you," the person who decided to observe fast tied on one of his feet a black thread and on the other a white thread. And he went on eating and drinking till he could distinguish (between their colour) on seeing them. It was after this that Allah reveal- ed (the words): min al-fajr. And they (the Muslims) came to know that (the word khait) refers to the night and day.  

لَمَّا نَزَلَتْ هَذِهِ الآيَةُ { وَكُلُوا وَاشْرَبُوا حَتَّى يَتَبَيَّنَ لَكُمُ الْخَيْطُ الأَبْيَضُ مِنَ الْخَيْطِ الأَسْوَدِ} قَالَ فَكَانَ الرَّجُلُ إِذَا أَرَادَ الصَّوْمَ رَبَطَ أَحَدُهُمْ فِي رِجْلَيْهِ الْخَيْطَ الأَسْوَدَ وَالْخَيْطَ الأَبْيَضَ فَلاَ يَزَالُ يَأْكُلُ وَيَشْرَبُ حَتَّى يَتَبَيَّنَ لَهُ رِئْيُهُمَا فَأَنْزَلَ اللَّهُ بَعْدَ ذَلِكَ { مِنَ الْفَجْرِ} فَعَلِمُوا أَنَّمَا يَعْنِي بِذَلِكَ اللَّيْلَ وَالنَّهَارَ  

When this verse was revealed." Eat and drink till the white streak becomes distinct from the dark streak for you," the person who decided to observe fast tied on one of his feet a black thread and on the other a white thread. And he went on eating and drinking till he could distinguish (between their colour) on seeing them. It was after this that Allah reveal- ed (the words): min al-fajr. And they (the Muslims) came to know that (the word khait) refers to the night and day. (Using translation from Muslim 1091b)  

لَمَّا نَزَلَتْ هَذِهِ الْآيَةُ {وَكُلُوا وَاشْرَبُوا حَتَّى يَتَبَيَّنَ لَكُمُ الْخَيْطُ الْأَبْيَضُ مِنَ الْخَيْطِ الْأَسْوَدِ} وَلَمْ يَنْزِلْ مِنَ الْفَجْرِ فَكَانَ رِجَالٌ إِذَا أَرَادُوا الصَّوْمَ رَبَطَ أَحَدُهُمْ فِي رِجْلَيْهِ الْخَيْطَ الْأَسْوَدَ وَالْخَيْطَ الْأَبْيَضَ فَلَا يَزَالُ يَأْكُلُ وَيَشْرَبُ حَتَّى يَتَبَيَّنَ لَهُ أَيُّهُمَا فَأَنْزَلَ اللهُ تَعَالَى بَعْدَ ذَلِكَ {مِنَ الْفَجْرِ} فَعَلِمُوا أَنَّمَا يَعْنِي بِذَلِكَ 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. (Using translation from Bukhārī 1917)   

نَزَلَتْ هَذِهِ الْآيَةُ {وَكُلُوا وَاشْرَبُوا حَتَّى يَتَبَيَّنَ لَكُمُ الْخَيْطُ الْأَبْيَضُ مِنَ الْخَيْطِ الْأَسْوَدِ} [البقرة 187] وَلَمْ يَنْزِلْ {مِنَ الْفَجْرِ} [البقرة 187] فَكَانَ رِجَالٌ إِذَا أَرَادُوا الصَّوْمَ رَبَطَ أَحَدُهُمْ فِي رِجْلِهِ الْخَيْطَ الْأَبْيَضَ وَالْخَيْطَ الْأَسْوَدَ وَلَا يَزَالُ يَأْكُلُ وَيُشْرَبُ حَتَّى يَتَبَيَّنَ لَهُ رُؤْيَتُهُمَا فَأَنْزَلَ اللهُ تَبَارَكَ وَتَعَالَى بَعْدَ ذَلِكَ {مِنَ الْفَجْرِ} [البقرة 187] فَعَلِمُوا أَنَّمَا يَعْنِي بِذَلِكَ اللَّيْلَ وَالنَّهَارَ