Musnad Ahmad 1153:2270

'Abd al-Wahhab narrated to us, from Sa'id, from Qatadah, from 'Abdullah ibn Rabah, that he entered upon 'Aishah and said: "I wish to ask you about something, but I am shy before you." She said: "Ask whatever occurs to you, for I am only your mother." I said: "O Mother of the Believers, what makes Ghusl (ritual bathing) obligatory?" She said: "When the two circumcised parts meet, Janabah (major ritual impurity) becomes obligatory." Qatadah used to follow this Hadith by saying that 'Aishah said: "I and the Messenger of Allah, peace and blessings of Allah be upon him, did that, and we performed Ghusl." I do not know if this is part of the Hadith or if Qatadah was saying it.