Musannaf Ibn Abi Shayba 35:408

Shababa ibn Sawwar narrated to us, from Layth ibn Sa'd, from Ibn Shihab, from 'Abd al-Rahman ibn Ka'b ibn Malik, that Jabir ibn 'Abd Allah informed him that the Prophet, peace and blessings of Allah be upon him, used to combine two men from the martyrs of Uhud in one grave. And he ordered that they be buried in their blood, and he did not perform the funeral prayer over them, and they were not washed.