Musannaf Ibn Abi Shaybah

Ibrahim narrates that Hazrat Ali (may Allah be pleased with him) made three men, who drowned while traveling in a boat, heirs to one another, even though their mother was alive, and made their mother the heir of one-sixth from each of them. Then, from the inheritance that was given to each of the brothers from the other, one-third of the wealth was given to the mother, and the remaining wealth was given to the 'asaba (residuary heirs).