Abdullah bin Rabah says that once he presented himself before Hazrat Aisha (may Allah be pleased with her) and said, "I want to ask you something, but I feel shy." She replied, "You may ask whatever you wish, for I am your mother." I said, "O Mother of the Believers! What makes ghusl (ritual bath) obligatory?" She replied, "When the private parts meet each other, ghusl becomes obligatory." After narrating this hadith, Qatadah would also say that Hazrat Aisha (may Allah be pleased with her) said, "And the Prophet (peace and blessings be upon him) used to do so, so we would perform ghusl." Now I do not know whether this sentence is part of the hadith or the statement of Qatadah.