Sahih Muslim Book - 13, Hadith - 2535

حَدَّثَنِى مُحَمَّدُ بْنُ سَهْلٍ التَّمِيمِىُّ وَأَبُو بَكْرِ بْنُ إِسْحَاقَ قَالاَ حَدَّثَنَا ابْنُ أَبِى مَرْيَمَ أَخْبَرَنَا أَبُو غَسَّانَ حَدَّثَنِى أَبُو حَازِمٍ عَنْ سَهْلِ بْنِ سَعْدٍ - رضى الله عنه - قَالَ لَمَّا نَزَلَتْ هَذِهِ الآيَةُ (وَكُلُوا وَاشْرَبُوا حَتَّى يَتَبَيَّنَ لَكُمُ الْخَيْطُ الأَبْيَضُ مِنَ الْخَيْطِ الأَسْوَدِ) قَالَ فَكَانَ الرَّجُلُ إِذَا أَرَادَ الصَّوْمَ رَبَطَ أَحَدُهُمْ فِى رِجْلَيْهِ الْخَيْطَ الأَسْوَدَ وَالْخَيْطَ الأَبْيَضَ فَلاَ يَزَالُ يَأْكُلُ وَيَشْرَبُ حَتَّى يَتَبَيَّنَ لَهُ رِئْيُهُمَا فَأَنْزَلَ اللَّهُ بَعْدَ ذَلِكَ (مِنَ الْفَجْرِ) فَعَلِمُوا أَنَّمَا يَعْنِى بِذَلِكَ اللَّيْلَ وَالنَّهَارَ.

It was narrated that Sahl bin Sa’d [may Allah be pleased with him] 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) – if a man wanted to fast, he would tie a white thread to one foot an a black thread to the other, then he would carry on eating and drinking until he could tell them apart when he saw them. Then after that, Allah revealed: “of dawn” then they realized that what was meant by that was night and day.”