Musannaf Ibn Abi Shayba 9:821

Abu Mu’awiyah narrated to us, from al-Shaybani, from al-Sha’bi: Umar was asked about a man who was absent from his wife, and it reached her that he had died, so she married. Then the first husband came. Umar said: "The first husband is given the choice between the dowry and his wife. If he chooses the dowry, he leaves her with the other husband. And if he wishes, he chooses his wife." And Ali said: "She is entitled to the dowry for what the other [man] made lawful of her private parts, and he is separated from her. Then she observes a waiting period of three menstrual cycles, then she is returned to the first."