The companions of Muhammad ﷺ used to observe this rule, if a man was fasting and the time to break the fast arrived, but he fell asleep before eating, he could not eat for the rest of that night nor through the following day until evening came again.Qays bin Sirmah Ansari was fasting one day, and when the time to break the fast came, he went to his wife and asked her, "Do you have any food?"She replied, "No, but let me go out and find something for you."He had been working all day, and sleep overcame him. When his wife returned and saw him asleep, she said, "What a loss for you!"By midday he had fainted. This was mentioned to the Prophet ﷺ, and then this verse was revealed, "It has been made lawful for you on the night of the fast to go in to your wives" [Baqarah 2:187]. They rejoiced greatly at this. Then there was revealed, "And eat and drink until the white thread becomes distinct to you from the black thread" [Baqarah 2:187].
كان أصحاب محمد ﷺ إذا كان الرجل صائما فحضر الإفطار فنام قبل أن يفطر لم يأكل ليلته ولا يومه حتى يمسي، وإن قيس بن صرمة الأنصاري كان صائما، فلما حضر الإفطار أتى أهله فقال لها: أعندك طعام؟ قالت: لا، ولكن أنطلق فأطلب لك، وكان يومه يعمل، فغلبته عيناه، فجاءت امرأته، فلما رأته قالت: خيبة لك، فلما انتصف النهار غشي عليه، فذكر ذلك للنبي ﷺ فنزلت هذه الآية: {أحل لكم ليلة الصيام الرفث إلى نسا إنكم} ففرحوا بها فرحا شديدا، ونزلت: {وكلوا واشربوا حتى يتبين لكم الخيط الأبيض من الخيط الأسود}