Allah said:

﴿وَكُلُواْ وَاشْرَبُواْ حَتَّى يَتَبَيَّنَ لَكُمُ الْخَيْطُ الأَبْيَضُ مِنَ الْخَيْطِ الأَسْوَدِ مِنَ الْفَجْرِ ثُمَّ أَتِمُّواْ الصِّيَامَ إِلَى الَّيْلِ﴾

(...and eat and drink until the white thread (light) of dawn appears to you distinct from the black thread (darkness of night), then complete your fast till the nightfall.)

Allah has allowed eating and drinking, along with having sexual intercourse, as we have stated, during any part of the night until the light of dawn is distinguished from the darkness of the night. Allah has described that time as `distinguishing the white thread from the black thread.' He then made it clearer when He said:

﴿مِنَ الْفَجْرِ﴾

(of dawn.)

As stated in a Hadith that Imam Abu `Abdullah Al-Bukhari recorded, Sahl bin Sa`d said, "When the following verse was revealed:

﴿وَكُلُواْ وَاشْرَبُواْ حَتَّى يَتَبَيَّنَ لَكُمُ الْخَيْطُ الأَبْيَضُ مِنَ الْخَيْطِ الأَسْوَدِ﴾

(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 to them that it meant (the darkness of) night and (the light of) day. ''

Al-Bukhari recorded that Ash-Sha`bi said that `Adi said, "I took two strings, one black and the other white and kept them under my pillow and went on looking at them throughout the night, but could not make any distinction between the two. So, the next morning I went to Allah's Messenger and told him the whole story. He said:

«إِنَّ وِسَادَكَ إِذًا لَعَرِيضٌ، أَنْ كَانَ الْخَيْطُ الْأَبْيَضُ وَالْأَسْوَدُ تَحْتَ وِسَادَتِك»

(Your pillow is very wide if the white and black threads are under it!) Some wordings for this Hadith read,

«إِنَّكَ لَعَرِيضُ الْقَفَا»

(Your Qafa (back side of your neck) is wide!)

Some people said that these words meant that `Adi was not smart. This is a weak opinion. The narration that Al-Bukhari collected explains this part of the Hadith. Al-Bukhari recorded that `Adi bin Hatim narrated: I said, "O Messenger of Allah! What is the white thread from the black thread Are they actual threads'' He said:

«إنَّكَ لَعَرِيضُ الْقَفَا أَنْ أَبْصَرْتَ الْخَيْطَيْنِ، ثُمَّ قَالَ: لَا بَلْ هُوَ سَوَادُ اللَّيْلِ وَبَيَاضُ النَّهَار»

(Your Qafa is wide if you see the two threads. Rather, they are the blackness of the night and the whiteness of the daylight.)