Hadith: A woman whose husband is missing and whose whereabouts is unknown to her must wait for four years and then observe the ‘Iddah (of a widow) for four months and ten days. After that, she is allowed to marry

عن عمر بن الخطاب أنه قال: «أَيُّمَا امرأة فَقَدَتْ زوجها فلم تَدْرِ أين هو؟ فإنها تَنْتَظِرُ أَرْبَعَ سِنِينَ، ثم تَعْتَدُّ أَرْبَعَةَ أشهر وعشرا ثم تَحِلُّ».
[صححه الحافظ ابن حجر] - [رواه مالك]

‘Umar ibn al-Khattāb (may Allah be pleased with him) said: “A woman whose husband is missing and whose whereabouts is unknown to her must wait for four years and then observe the ‘Iddah (of a widow) for four months and ten days. After that, she is allowed to marry.”
[No judgment by Shaykh Al-Albaani is available] - [Maalik]

Explanation

In this tradition, ‘Umar ibn al-Khattāb (may Allah be pleased with him) explains that any woman whose husband went missing must wait for four years, starting from the date when she lost him. Then she should observe the ‘Iddah of a widow for four months and ten days. After that, she is allowed to marry. Muslim jurists said that this tradition and similar ones apply to the husband whose death is almost certain, such as the one who went missing in a war, drowned at sea, etc.