Al-Kāfi - Volume 3 > Bandages, Injuries, One’s Suffering from Smallpox and or Experiencing Carnal Relations
Hadith #3

3ـ عِدَّةٌ مِنْ أَصْحَابِنَا عَنْ أَحْمَدَ بْنِ مُحَمَّدٍ عَنْ عَلِيِّ بْنِ أَحْمَدَ رَفَعَهُ عَنْ أَبِي عَبْدِ الله (عَلَيْهِ الْسَّلام) قَالَ سَأَلْتُهُ عَنْ مَجْدُورٍ أَصَابَتْهُ جَنَابَةٌ قَالَ إِنْ كَانَ أَجْنَبَ هُوَ فَلْيَغْتَسِلْ وَإِنْ كَانَ احْتَلَمَ فَلْيَتَيَمَّمْ.

3. A number of our people have narrated from Ahmad ibn Muhammad from Muhammad ibn Ali from Ahmad who in a marfu‘ manner has said the following: “I once asked abu ‘Abd Allah, ‘Alayhi al-Salam, about one with a condition of smallpox who experiences sexual relation. He (the Imam) said, ‘If one has experienced sexual relation, he must take Ghusl (bath), but if it is wet dream, there is Tayammum for it.’”