كتاب الصيام
13  
The Book of Fasting
(14)
Chapter: The strict prohibition of intercourse during the day in Ramadan for one who is fasting; And the obligation of offering major expiation and the definition thereof; And that it is obligatory for both the one who can afford it and the one who cannot afford it, and it remains an obligation for the one who cannot afford it until he has the means
(14)
باب تَغْلِيظِ تَحْرِيمِ الْجِمَاعِ فِي نَهَارِ رَمَضَانَ عَلَى الصَّائِمِ وَوُجُوبِ الْكَفَّارَةِ الْكُبْرَى فِيهِ وبيانها وأنها تجب على الموسر والمعسر وتثبت في ذمة المعسر حتى يستطيع
Sahih Muslim 1112 a

'A'isha (Allah be pleased with her) reported that a person came to the Messenger of Allah (ﷺ) and said:

I am burnt, whereupon the Messenger of Allah (ﷺ) said: How is it? He (the person) said: I had intercourse with my wife during the day in Ramadan. Upon this (the Holy Prophet) said: Give charity, give charity. He (the person) said: There is nothing with me. He commanded him to sit down, (In the meanwhile) there were brought to him (to the Holy Prophet) two baskets containing eatables, whereupon the Messenger of Allah (ﷺ) told him to give them as sadaqa.
حَدَّثَنَا مُحَمَّدُ بْنُ رُمْحِ بْنِ الْمُهَاجِرِ، أَخْبَرَنَا اللَّيْثُ، عَنْ يَحْيَى بْنِ سَعِيدٍ، عَنْ عَبْدِ، الرَّحْمَنِ بْنِ الْقَاسِمِ عَنْ مُحَمَّدِ بْنِ جَعْفَرِ بْنِ الزُّبَيْرِ، عَنْ عَبَّادِ بْنِ عَبْدِ اللَّهِ بْنِ الزُّبَيْرِ، عَنْ عَائِشَةَ، - رضى الله عنها - أَنَّهَا قَالَتْ جَاءَ رَجُلٌ إِلَى رَسُولِ اللَّهِ صلى الله عليه وسلم فَقَالَ احْتَرَقْتُ ‏.‏ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ لِمَ ‏"‏ ‏.‏ قَالَ وَطِئْتُ امْرَأَتِي فِي رَمَضَانَ نَهَارًا ‏.‏ قَالَ ‏"‏ تَصَدَّقْ تَصَدَّقْ ‏"‏ ‏.‏ قَالَ مَا عِنْدِي شَىْءٌ ‏.‏ فَأَمَرَهُ أَنْ يَجْلِسَ فَجَاءَهُ عَرَقَانِ فِيهِمَا طَعَامٌ فَأَمَرَهُ رَسُولُ اللَّهِ صلى الله عليه وسلم أَنْ يَتَصَدَّقَ بِهِ ‏.‏
Reference : Sahih Muslim 1112a
In-book reference : Book 13, Hadith 107
USC-MSA web (English) reference : Book 6, Hadith 2463
  (deprecated numbering scheme)