Al-Kāfi - Volume 3 > Reason Necessitating Ghusl (bath) for Men and Women
Hadith #2

2ـ عِدَّةٌ مِنْ أَصْحَابِنَا عَنْ أَحْمَدَ بْنِ مُحَمَّدِ بْنِ عِيسَى عَنْ مُحَمَّدِ بْنِ إِسْمَاعِيلَ قَالَ سَأَلْتُ الرِّضَا (عَلَيْهِ الْسَّلام) عَنِ الرَّجُلِ يُجَامِعُ الْمَرْأَةَ قَرِيباً مِنَ الْفَرْجِ فَلاَ يُنْزِلاَنِ مَتَى يَجِبُ الْغُسْلُ فَقَالَ إِذَا الْتَقَى الْخِتَانَانِ فَقَدْ وَجَبَ الْغُسْلُ فَقُلْتُ الْتِقَاءُ الْخِتَانَيْنِ هُوَ غَيْبُوبَةُ الْحَشَفَةِ قَالَ نَعَمْ.

2. A number of our people have narrated from Ahmad ibn Muhammad ibn ‘Isa from Muhammad ibn ’Isma‘il who has said the following: “I once asked al-Rida, ‘Alayhi al-Salam, about a man who performs sexual relation with woman near her vagina but does not discharge, if Ghusl (bath) becomes necessary. He (the Imam) said, ‘When private parts touch each other Ghusl (bath) becomes necessary.’ I (the narrator) then asked, ‘Is touching of private parts disappearance of glans penis?’ He (the Imam) replied, ‘Yes, it is such.’”