Maliks Muwatta Book 29, Hadith Number 52.
Yahya related to me from Malik from Yahya ibn Said from Said ibn al-Musayyab that Umar ibn al-Khattab said, “The woman who loses her husband and does not know where he is, waits for four years, then she does idda for four months, and then she is free to marry.”
Malik said, “If she marries after her idda is over, regardless of whether the new husband has consummated the marriage or not, her first husband has no means of access to her.”
Malik said, “That is what is done among us and if her husband reaches her before she has remarried, he is more entitled to her.”
Malik said that he had seen people disapproving of someone who said that one of the people (of knowledge) attributed to Umar ibn al-Khattab that he said, “Her first husband chooses when he comes either her bride-price or his wife.”
Malik said, “I have heard that Umar ibn al-Khattab, speaking about a woman whose husband divorced her while he was absent from her, and then he took her back and the news of his taking her back had not reached her, while the news of his divorcing her had, and so she had married again, said, ‘Her first husband who divorced her has no means of access to her whether or not the new husband has consummated the marriage.'”
Malik said, “This is what I like the best of what I heard about the missing man.”