It is narrated from Hazrat Aisha (may Allah be pleased with her) that the Prophet (peace and blessings be upon him) said: When the private part meets the private part, ghusl becomes obligatory.
Hadith Referenceمسند احمد / مسند النساء / 25037
Hadith Gradingحکم دارالسلام:حديث صحيح، وهذا إسناد ضعيف لضعف على بن زيد بن جدعان