Bukhari Collection: ::Book Number 7:: Hadith # 190
Narrated `Aisha:A man divorced his wife and she married another man who proved to be impotent and divorced her. She could not get her satisfaction from him, and after a while he divorced her. Then she came to the Prophet and said, "O Allah's Apostle! My first husband divorced me and then I married another man who entered upon me to consummate his marriage but he proved to be impotent and did not approach me except once during which he benefited nothing from me. Can I remarry my first husband in this case?" Allah's Apostle said, "It is unlawful to marry your first husband till the other husband consummates his marriage with you."