The inheritance of those whose order of death is unknown

Sayyiduna Zayd bin Thabit (may Allah be pleased with him) said: When the people of al-Yamamah were martyred, Sayyiduna Abu Bakr (may Allah be pleased with him) ordered me that the living be made heirs of the dead, and that those dead not be made heirs of one another.

Sayyiduna Zayd bin Thabit (may Allah be pleased with him) said: Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) gave me an order during the plague of Amwas, when a (whole) tribe had perished and another people had been made their heirs. He said that I should make their living people the heirs of the dead, and not make the dead heirs of one another.
The Shaykh (al-Bayhaqi) (may Allah have mercy on him) said: It is reported from Sayyiduna Umar (may Allah be pleased with him) that he made some of them heirs of others in their inherited wealth, and in one narration he said to Sayyiduna Ali (may Allah be pleased with him): Make these people heirs; so he made them heirs of their inherited wealth. It is narrated from Qatadah (may Allah have mercy on him) that he made the people of the plague of Amwas heirs of one another: when the hand and foot of one was on top of the other, he made the upper one an heir of the lower one, and did not make the lower one an heir of the upper one.
These two reports are munqati' (with broken chains). It has also been said: from Qatadah, from Raja' bin Haywah, from Qabisah bin Dhu'ayb, from Umar, and that too is munqati'. So what we have narrated from Umar is more likely correct, and Allah knows best.

Kharijah bin Zayd narrates from his father Zayd bin Thabit (may Allah be pleased with him) that he said, regarding people who would inherit from one another but who died by drowning or perished from some other cause, and it was not known which of them died first: They will not inherit from one another.

Ibn Abi al-Zinad (may Allah have mercy on him) narrates from the jurists of the people of Madinah that they used to say: Any people who would inherit from one another and who die, whether from something collapsing on them, by drowning, by burning or from any other cause, and it is not known whose death came before the other's, neither inherit from one another nor exclude (others) as hajib (blockers). This was the view of Zayd bin Thabit (may Allah be pleased with him), and Umar bin Abd al-Aziz (may Allah have mercy on him) ruled accordingly.

Ja'far bin Muhammad (may Allah have mercy on him) narrates from his father that Sayyidah Umm Kulthum bint Ali (may Allah be pleased with her) and her son Zayd (may Allah be pleased with him) both died on the same day, and it could not be known whose death came first. So neither did Sayyidah Umm Kulthum (may Allah be pleased with her) inherit from him, nor did Zayd (may Allah be pleased with him) inherit from Sayyidah Umm Kulthum (may Allah be pleased with her). And the people of Siffin were not made heirs of one another, and the people of al-Harrah were not made heirs of one another.

Abu al-Zinad said: A trustworthy person informed me that when the people of al-Harrah were afflicted, the one who judged their cases was Sayyiduna Zayd bin Thabit (may Allah be pleased with him), and among the people on that day were Companions of the Prophet (peace and blessings be upon him) and their children as well.

Umarah bin Hazn narrates from his father that Sayyiduna Ali (may Allah be pleased with him) made the living (relatives) of those killed in the Battle of the Camel their heirs.

Hazn bin Bashir al-Khath'ami narrates from his father that Sayyiduna Ali (may Allah be pleased with him) made a man and his son, or two brothers, who were killed at Siffin, and of whom it was not known which had died before the other, heirs: he made some of them heirs of others. [Da'if]

Rabi'ah bin Abi Abd al-Rahman (may Allah have mercy on him) narrates from their scholars that those killed on the day of the Camel, the day of Siffin and the day of al-Harrah were not made heirs of one another; then on the day of Qudayd too, none of those who were killed was made an heir of his companion, except where it was known who had died first.
Imam Malik (may Allah have mercy on him) said: There is no disagreement among us on this matter, nor any doubt about it among any of the people of knowledge in our city. And our Imam Ahmad (may Allah have mercy on him) narrates from Iyas bin Abd al-Muzani that some of them are to be made heirs of others; but the view of the majority is more correct.