Musnad Ahmad 658:143

Aswad bin Amir and Abu Ahmad narrated to us, they said: Isra'il narrated to us, from Abu Ishaq, from Al-Bara', who said: When the companions of Muhammad, peace and blessings of Allah be upon him, were fasting and the time for breaking the fast arrived, if one of them slept before breaking his fast, he would not eat for the rest of his night nor his day until the evening. A certain man from the Ansar was fasting, and when the time for breaking the fast arrived, he came to his wife and said: "Do you have any food?" She said: "No, but I will go and seek some for you." His eyes overcame him (he fell asleep), and his wife came to him. When she saw him, she said: "Woe to you!" He woke up in the morning, and when it was midday, he fainted. This was mentioned to the Prophet, peace and blessings of Allah be upon him, and this verse was revealed: "It has been made permissible for you the night preceding fasting to go to your wives [for sexual relations]..." up to His saying: "...until the white thread of dawn becomes distinct to you from the black thread [of night]." Abu Ahmad said: And Qays bin Sirmah Al-Ansari came and slept, and he mentioned the same.