It was the custom among the companions of Muhammad that if any of them was fasting and the food was presented (for breaking his fast), but he slept before eating, he would not eat that night and the following day till sunset. Qais bin Sirma-al-Ansari was fasting and came to his wife at the time of Iftar (breaking one's fast) and asked her whether she had anything to eat. She replied, "No, but I would go and bring some for you." He used to do hard work during the day, so he was overwhelmed by sleep and slept. When his wife came and saw him, she said, "Disappointment for you." When it was midday on the following day, he fainted and the Prophet ﷺ was informed about the whole matter and the following verses were revealed: "You are permitted To go to your wives (for sexual relation) At the night of fasting." So, they were overjoyed by it. And then Allah also revealed: "And eat and drink Until the white thread Of dawn appears to you Distinct from the black thread (of the night)." (2.187) (Using translation from Bukhārī 1915)  

«كَانَ أَصْحَابُ مُحَمَّدٍ ﷺ إِذَا كَانَ الرَّجُلُ صَائِمًا فَحَضَرَ الْإِفْطَارُ فَنَامَ قَبْلَ أَنْ يُفْطِرَ لَمْ يَأْكُلْ لَيْلَتَهُ وَلَا يَوْمَهُ حَتَّى يُمْسِيَ»  

[Machine] I said, "O Messenger of Allah, I have placed a white thread and a black thread under my pillow, but I cannot distinguish between them." He said, "You have a wide bedspread, and that night is part of the day, as Allah says, 'And eat and drink until the white thread of dawn becomes distinct to you from the black thread. Then complete the fast until the night.' And do not approach them while you are in seclusion in the mosques. These are the limits set by Allah, so do not come near them.' Thus Allah makes clear His signs to the people that they may fear Him."  

قُلْتُ يَا رَسُولَ اللَّهِ لَقَدْ جَعَلْتُ تَحْتَ وِسَادَتِي خَيْطًا أَبْيَضَ وَخَيْطًا أَسْوَدَ فَمَا تَبَيَّنَ لِي شَيْءٌ قَالَ «إِنَّكَ لَعَرِيضُ الْوِسَادِ وَإِنَّمَا ذَلِكَ اللَّيْلُ مِنَ النَّهَارِ فِي قَوْلِهِ تَعَالَى» {وَكُلُوا وَاشْرَبُوا حَتَّى يَتَبَيَّنَ لَكُمُ الْخَيْطُ الْأَبْيَضُ مِنَ الْخَيْطِ الْأَسْوَدِ مِنَ الْفَجْرِ ثُمَّ أَتِّمُّوا الصِّيَامَ إِلَى اللَّيْلِ وَلَا تُبَاشِرُوهُنَّ وَأَنْتُمْ عَاكِفُونَ فِي الْمَسَاجِدِ تِلْكَ حُدُودُ اللَّهِ فَلَا تَقْرَبُوهَا كَذَالِكَ يُبَيِّنُ اللَّهُ آيَاتِهِ لِلنَّاسِ لَعَلَّهُمْ يَتُّقُونَ}