Musannaf Ibn Abi Shayba 36:235

Mu'tamir ibn Sulayman narrated to us, from Ma'mar, from al-Zuhri, from a man, from Jabir, that the Prophet (peace and blessings of Allah be upon him) commanded regarding the slain on the day of Uhud that they be buried in their blood, and that the one among them who had memorized the most Quran be placed forward, and that two be buried in one grave. He said: So I buried my father and my uncle in one grave.