Sayyiduna Ali (may Allah be pleased with him) said concerning the woman whose husband goes missing: when he comes back and the woman has remarried, then if he wishes he may divorce her, and if he wishes he may keep her, and the woman is not given a choice.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15562
It is narrated from Hanash that Ali (may Allah be pleased with him) said: What Umar (may Allah be pleased with him) said is nothing, meaning concerning the wife of the missing husband. She is the wife of the one who is absent until certain news of his death or certain news of his divorcing her reaches her. She is owed the dowry from this man because he made her private parts lawful for himself, and his marriage is void.
And Sa'id bin Jubayr (may Allah have mercy on him) narrated to us from Ali (may Allah be pleased with him) that Ali (may Allah be pleased with him) said: She is the wife of the first husband, whether the second one consummated the marriage with her or not.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15563
It is narrated from Ibn Shubrumah (may Allah have mercy on him) that Umar bin Abd al-Aziz (may Allah have mercy on him) wrote concerning the wife of the missing husband that she should wait and be patient.
A musnad hadith has also been narrated on this, but its chain contains someone whose hadith is not used as evidence.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15564
It is narrated from Mughirah bin Shu'bah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "The woman whose husband goes missing remains his wife until clear news about him comes to her."
Likewise Zakariyya bin Yahya al-Wasiti narrated it from Sawwar bin Mus'ab, and Sawwar is a da'if (weak) narrator.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15565