" 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