Musnad Ahmad 1153:1019

Waki’ narrated to us, Sufyan narrated to us, from Ali ibn Zayd ibn Jud’an, from Sa’id ibn al-Musayyib, from Aisha, she said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When the circumcision [of the man] passes the circumcision [of the woman], then Ghusl (ritual bathing) has become obligatory."