Al-Kāfi - Volume 3 > Wet Dreams of Men and Women
Hadith #3

3ـ عَلِيُّ بْنُ إِبْرَاهِيمَ عَنْ أَبِيهِ عَنْ حَمَّادِ بْنِ عِيسَى عَنْ حَرِيزٍ عَنْ زُرَارَةَ قَالَ إِذَا كُنْتَ مَرِيضاً فَأَصَابَتْكَ شَهْوَةٌ فَإِنَّهُ رُبَّمَا كَانَ هُوَ الدَّافِقَ لَكِنَّهُ يَجِي‏ءُ مَجِيئاً ضَعِيفاً لَيْسَ لَهُ قُوَّةٌ لِمَكَانِ مَرَضِكَ سَاعَةً بَعْدَ سَاعَةٍ قَلِيلاً قَلِيلاً فَاغْتَسِلْ مِنْهُ.

3. Ali ibn Ibrahim has narrated from his father from Hammad ibn ‘Isa from Hariz from Zurarah who has said the following: “He (the Imam) has said, ‘When you suffer from an illness and experience orgasm, it may take place with excitement, but if it takes place in a weak manner with less force because of your illness, in intervals little by little, in such case Ghusl (bath) is obligatory.’”