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.  

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  

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  

" 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).  

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 ﷻ { مِنَ الْفَجْرِ} فَبَيَّنَ ذَلِكَ  

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  

[Machine] "When this verse was revealed {Eat and drink until the white thread becomes distinct to you from the black thread of dawn}, I took two headbands, one black and the other white, and put them under my pillow. I raised them at night but could not determine the white from the black. So, I mentioned this to the Messenger of Allah ﷺ, and he said, 'O son of Hatim, if that is the case, then your pillow is too wide. Indeed, what Allah meant is to eat and drink until the whiteness of day becomes distinct from the darkness of night.'"  

لَمَّا نَزَلَتْ هَذِهِ الْآيَةُ {وَكُلُوا وَاشْرَبُوا حَتَّى يَتَبَيَّنَ لَكُمُ الْخَيْطُ الْأَبْيَضُ مِنَ الْخَيْطِ الْأَسْوَدِ مِنَ الْفَجْرِ} أَخَذْتُ عِقَالَيْنِ أَحَدُهُمَا أسْوَدُ وَالْآخَرُ أَبْيَضُ فَوَضَعْتُهُمَا تَحْتَ وِسَادَتِي فَرَفَعَتْهُمَا فِي اللَّيْلِ فَلَمْ أَتَبَيَّنْ الْأَبْيَضَ مِنَ الْأَسْوَدِ فَذَكَرْتُ ذَلِكَ لِرَسُولِ اللهِ ﷺ فَقَالَ يَا ابْنَ حَاتِمٍ إِنَّ وِسَادَكَ إِذًا لَعَرِيضٌ إِنَّمَا قَالَ كُلُوا وَاشْرَبُوا حَتَّى يَتَبَيَّنَ لَكُمْ بَيَاضُ النَّهَارِ مِنْ سَوَادِ اللَّيْلِ  

[Machine] When this verse was revealed, I took two ropes, one of them black and the other white, and placed them under my pillow. I raised them in the night but I could not distinguish the white from the black. I mentioned this to the Messenger of Allah ﷺ , and he said, "O son of Hatim, if your pillow had been truly wide, you would have been able to distinguish the white from the black. Allah only meant that you should eat and drink until the whiteness of the day can be distinguished from the darkness of the night."  

لَمَّا نَزَلَتْ هَذِهِ الْآيَةُ {وَكُلُوا وَاشْرَبُوا حَتَّى يَتَبَيَّنَ لَكُمُ الْخَيْطُ الْأَبْيَضُ مِنَ الْخَيْطِ الْأَسْوَدِ مِنَ الْفَجْرِ} أَخَذْتُ عِقَالَيْنِ أَحَدُهُمَا أسْوَدُ وَالْآخَرُ أَبْيَضُ فَوَضَعْتُهُمَا تَحْتَ وِسَادَتِي فَرَفَعَتْهُمَا فِي اللَّيْلِ فَلَمْ أَتَبَيَّنْ الْأَبْيَضَ مِنَ الْأَسْوَدِ فَذَكَرْتُ ذَلِكَ لِرَسُولِ اللهِ ﷺ فَقَالَ يَا ابْنَ حَاتِمٍ إِنَّ وِسَادَكَ إِذًا لَعَرِيضٌ إِنَّمَا قَالَ كُلُوا وَاشْرَبُوا حَتَّى يَتَبَيَّنَ لَكُمْ بَيَاضُ النَّهَارِ مِنْ سَوَادِ اللَّيْلِ  

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  

[Machine] This verse was revealed {and eat and drink until the white thread of dawn becomes distinct to you from the black thread} [Surah Al-Baqarah 187], and it did not reveal {of dawn} [Surah Al-Baqarah 187]. He said, "There were men among us who, when they wanted to fast, would tie a white thread and a black thread around one of their legs. He would continue to eat and drink until he could distinguish between the two threads. Then Allah, the Most High, revealed after that {of dawn} [Surah Al-Baqarah 187], so they knew that it only meant the night and the day." Ibn Abi Maryam said, "Ibn Abi Hazim narrated to me from his father from Sahl bin Saad in a similar manner."  

نَزَلَتْ هَذِهِ الْآيَةُ {وَكُلُوا وَاشْرَبُوا حَتَّى يَتَبَيَّنَ لَكُمُ الْخَيْطُ الْأَبْيَضُ مِنَ الْخَيْطِ الْأَسْوَدِ} [البقرة 187] وَلَمْ يَنْزِلْ {مِنَ الْفَجْرِ} [البقرة 187] قَالَ وَكَانَ رِجَالٌ إِذَا أَرَادُوا الصَّوْمَ رَبَطَ أَحَدُهُمْ فِي رِجْلَيْهِ الْخَيْطَ الْأَبْيَضَ وَالْخَيْطَ الْأَسْوَدَ فَلَا يَزَالُ يَأْكُلُ وَيَشْرَبُ حَتَّى يَتَبَيَّنَ لَهُ رُؤْيَتُهُمَا فَأَنْزَلَ اللهُ تَعَالَى بَعْدَ ذَلِكَ {مِنَ الْفَجْرِ} [البقرة 187] فَعَلِمُوا أَنَّهُ إِنَّمَا يَعْنِي بِذَلِكَ اللَّيْلَ وَالنَّهَارَ قَالَ ابْنُ أَبِي مَرْيَمَ وَحَدَّثَنِي ابْنُ أَبِي حَازِمٍ عَنْ أَبِيهِ عَنْ سَهْلِ بْنِ سَعْدٍ بِنَحْوِهِ