Fiqh-us-Sunnah - 4.48

Hadith 4.48.1

Person Surviving a Battle but Later Dying of Wounds

If one is wounded in a battle, but survives, remains in a stable condition for a while, and then dies, he should be washed, and a funeral prayer should be offered for him, even though he may be a martyr. The Prophet, peace be upon him, washed Sa'd ibn Mu'adh, and offered a funeral prayer for him, after he died of his hand wounds. Sa'd was taken to the mosque, where he remained for a few days, and then he died as a martyr because of his infected wound.
If on the other hand, a wounded fighter does not survive in a stable condition, or he just talked or drank water and thereafter died, he is not to be washed or offered a funeral prayer.
The author of Al-Mughni states: "It is recorded in Futuh Al-Sham, that a man said: "I took some water to give a drink to my cousin if he were still alive after some injuries in the battle. On the way, I passed by Al-Harith ibn Hisham, who was also wounded in the same battle. I wanted to give him a drink, but he noticed that another wounded man was looking toward him for a drink. At this he pointed that I should first give this man a drink. I went toward him to give him a drink, but he also found another man looking to him. So he gestured that I should first give him the drink. Thus they all died. None of them was washed or offered a funeral prayer although they all died after the battle.