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 d

This hadith has been narrated on the authority of Zuhri with the same chain of transmitters that a person broke fast in Ramadan whereupon the Messenger of Allah (ﷺ) commanded him to free a slave (as an atonement), and the rest of the hadith is the same as narrated by Ibn Uyaina.

وَحَدَّثَنَا مُحَمَّدُ بْنُ رَافِعٍ، حَدَّثَنَا إِسْحَاقُ بْنُ عِيسَى، أَخْبَرَنَا مَالِكٌ، عَنِ الزُّهْرِيِّ، بِهَذَا الإِسْنَادِ أَنَّ رَجُلاً، أَفْطَرَ فِي رَمَضَانَ فَأَمَرَهُ رَسُولُ اللَّهِ صلى الله عليه وسلم أَنْ يُكَفِّرَ بِعِتْقِ رَقَبَةٍ ‏.‏ ثُمَّ ذَكَرَ بِمِثْلِ حَدِيثِ ابْنِ عُيَيْنَةَ ‏.‏
Reference : Sahih Muslim 1111d
In-book reference : Book 13, Hadith 104
USC-MSA web (English) reference : Book 6, Hadith 2460
  (deprecated numbering scheme)