It is narrated from Hazrat Aisha that the Prophet (peace 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 / 24817
Hadith Gradingحکم دارالسلام:حديث صحيح، وهذا إسناد ضعيف لأجل على بن زيد