Chapter on I'tikaf (spiritual Retreat)
Man Lā Yaḥḍuruh al-Faqīh|Volume 2|Book 2|Chapter 38
وَ رَوَى اِبْنُ اَلْمُغِيرَةِ عَنْ سَمَاعَةَ قَالَ: سَأَلْتُ أَبَا عَبْدِ اَللَّهِ عَلَيْهِ اَلسَّلاَمُ عَنْ مُعْتَكِفٍ وَاقَعَ أَهْلَهُ فَقَالَ «هُوَ بِمَنْزِلَةِ مَنْ أَفْطَرَ يَوْماً مِنْ شَهْرِ رَمَضَانَ ».
It is narrated by Ibn Al-Mughira, from Sama'ah, who said: "I asked Abu Abdullah عليه السلام about a person performing i'tikaf who engaged in sexual relations with their spouse. Imam عليه السلام said: 'They are like one who has broken their fast on a day of Month of Ramadan.'"
- Ṣaḥīḥ al-Faqīh, p. 112