Imam Malik's Muwatta, Chapter: Jihad

Yahya related to me from Malik that he had heard the people of knowledge say that martyrs in the way of Allah were not washed, nor were any of them prayed over. They were buried in the garments in which they were slain. Malik said, "That is the sunna for someone who is killed on the battleground and is not reached until he is already dead. Someone who is carried off and lives for as long as Allah wills after it, is washed and prayed over as was Umar ibn al-Khattab."