Al-Kāfi - Volume 3 > Reason Necessitating Ghusl (bath) for Men and Women
Hadith #4

4ـ عَلِيُّ بْنُ إِبْرَاهِيمَ عَنْ أَبِيهِ عَنِ ابْنِ أَبِي عُمَيْرٍ عَنْ حَمَّادِ بْنِ عُثْمَانَ عَنْ عُبَيْدِ الله الْحَلَبِيِّ قَالَ سَأَلْتُ أَبَا عَبْدِ الله (عَلَيْهِ الْسَّلام) عَنِ الْمُفَخِّذِ عَلَيْهِ غُسْلٌ قَالَ نَعَمْ إِذَا أَنْزَلَ.

4. Ali ibn Ibrahim has narrated from his father from ibn abu ‘Umayr from Hammad ibn ‘Uthman from ‘Ubayd Allah al-Halabiy who has said the following: “I once asked abu ‘Abd Allah, Alayhi al-Salam, about one’s foreplay and Ghusl (bath). He (the Imam) said, ‘There is Ghusl (bath) obligatory if semen discharge takes place.’”