Someone asked Hazrat Aisha (may Allah be pleased with her) about when ghusl becomes obligatory due to relations between a man and a woman. She replied that the Prophet (peace and blessings be upon him) would perform ghusl when ejaculation occurred.
Hadith Referenceمسند احمد / مسند النساء / 25201
Hadith Gradingحکم دارالسلام:إسناده ضعيف لابهام الرجل من بني سوءة