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 1111 c

Abu Huraira reported that a person had intercourse with his wife during Ramadan (while fasting). He asked for the religious verdict (about it) from the Messenger of Allah (ﷺ), whereupon he (the Holy Prophet) said:

Can you find a slave (to grant him freedom)? He said: No. He (the Prophet again) said: Can you afford to observe fasts for two (consecutive) months? He said: No. He (the Holy Prophet) said: Then feed sixty poor men.
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، وَمُحَمَّدُ بْنُ رُمْحٍ، قَالاَ أَخْبَرَنَا اللَّيْثُ، ح وَحَدَّثَنَا قُتَيْبَةُ، حَدَّثَنَا لَيْثٌ، عَنِ ابْنِ شِهَابٍ، عَنْ حُمَيْدِ بْنِ عَبْدِ الرَّحْمَنِ بْنِ عَوْفٍ، عَنْ أَبِي هُرَيْرَةَ، - رضى الله عنه - أَنَّ رَجُلاً، وَقَعَ بِامْرَأَتِهِ فِي رَمَضَانَ فَاسْتَفْتَى رَسُولَ اللَّهِ صلى الله عليه وسلم عَنْ ذَلِكَ فَقَالَ ‏"‏ هَلْ تَجِدُ رَقَبَةً ‏"‏ ‏.‏ قَالَ لاَ ‏.‏ قَالَ ‏"‏ وَهَلْ تَسْتَطِيعُ صِيَامَ شَهْرَيْنِ ‏"‏ ‏.‏ قَالَ لاَ ‏.‏ قَالَ ‏"‏ فَأَطْعِمْ سِتِّينَ مِسْكِينًا ‏"‏ ‏.‏
Reference : Sahih Muslim 1111c
In-book reference : Book 13, Hadith 103
USC-MSA web (English) reference : Book 6, Hadith 2459
  (deprecated numbering scheme)