Musannaf Ibn Abi Shayba 36:219

Shababa narrated to us, he said: Layth ibn Sa’d narrated to us, 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 a single garment. Then he would say, "Which of them was more learned in the Qur’an?" If one of them was pointed out to him, he would place him forward in the *lahd* (niche of the grave). He said, "I am a witness over these people on the Day of Resurrection." And he ordered that they be buried with their blood, and he did not perform the funeral prayer over them, nor were they washed.