It is narrated from Sayyiduna Ibn Amr (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: "When the private parts meet and the man's private part disappears (in intercourse), then ghusl becomes obligatory."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 6670
Hadith Gradingحکم دارالسلام:صحيح لغيره، وهذا إسناد ضعيف لضعف حجاج بن أرطاة