It is narrated from Hazrat Aisha (may Allah be pleased with her) that the Prophet (peace and blessings be upon him) said: When a man sits between the four corners of a woman and the private parts meet, then ghusl becomes obligatory.
Hadith Referenceمسند احمد / مسند النساء / 24206
Hadith Gradingحکم دارالسلام:حديث صحيح وهذا إسناد ضعيف لضعف على بن زيد