Al-Kāfi - Volume 3 > Reason Necessitating Ghusl (bath) for Men and Women
Hadith #6

6ـ مُحَمَّدُ بْنُ يَحْيَى عَنْ أَحْمَدَ بْنِ مُحَمَّدٍ عَنْ مُحَمَّدِ بْنِ إِسْمَاعِيلَ بْنِ بَزِيعٍ قَالَ سَأَلْتُ الرِّضَا (عَلَيْهِ الْسَّلام) عَنِ الرَّجُلِ يُجَامِعُ الْمَرْأَةَ فِيمَا دُونَ الْفَرْجِ وَتُنْزِلُ الْمَرْأَةُ عَلَيْهَا غُسْلٌ قَالَ نَعَمْ.

6. Muhammad ibn Yahya has narrated from Ahmad ibn Muhammad from Muhammad ibn Tsma‘il ibn Bazi‘ who has said the following: “I once asked al-Rida, Alayhi al-Salam, about a man who allows intercourse with a woman to take place but not in vagina, and she feels discharges, is Ghusl (bath) necessary on her? He (the Imam) said, ‘Yes, Ghusl (bath) is necessary for her.’”