Sahih Muslim Book - 13, Hadith - 2534

حَدَّثَنَا عُبَيْدُ اللَّهِ بْنُ عُمَرَ الْقَوَارِيرِىُّ حَدَّثَنَا فُضَيْلُ بْنُ سُلَيْمَانَ حَدَّثَنَا أَبُو حَازِمٍ حَدَّثَنَا سَهْلُ بْنُ سَعْدٍ قَالَ 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 عَزَّ وَجَلَّ (مِنَ الْفَجْرِ) فَبَيَّنَ ذَلِكَ.

Sahl bin sa’d said: “When this verse was revealed –“And eat and drink until the white thread appears to you distinct from the black thread”(Al-Baqarah 2:187)- a man would take a white thread and a black thread and he would eat until he could tell them apart, until Allah, the Mighty and Sublime, revealed: “of dawn” then it became clear.”