Musannaf Ibn Abi Shayba 26:308

Hushaym narrated to us, from Mughira, who said: Qatan ibn Abdullah al-Dabbi informed me that a woman boarded [a boat] on the Euphrates, and her son was with her, and they both drowned together. It was not known which of them died before the other. So we came to Shurayh and informed him of that. He said: "Make each one of them inherit from the other, and do not return anything of what one of them inherited from the other to either of them."