Fiqh-us-Sunnah Volume 4, Funerals and Dhikr, Fiqh 4.028A.

It is not necessary for a Muslim to wash a disbeliever’s body. Some scholars, however, consider it permissible. According to the Maliki and Hanbali schools, a Muslim may not wash a disbeliever’ s body even if he is one of his near of kin. Similarly a Muslim is not permitted to shroud him or bury him unless it is feared that the body would decompose because of weather conditions, or that it would attract predatory beasts, etc. In such a case, he may be buried in a hole and covered with dust. This opinion is based on a tradition transmitted by Ahmad, Abu Daw’ud, Nasa’i and Al-Baihaqi on the authority of Ali who said: “I said to Allah’s Messenger ‘Your uncle, the old misguided man, has passed away.’ The Prophet, peace be upon him, said: ‘Go and bury your father and do not do anything else until you return to me’.” ‘Ali said: “I did as he had told me and went back to him. The Prophet, peace be upon him, ordered me to take a bath, and then he prayed for me.”

Ibn al-Munzhir says there is nothing specific reported from the Prophet, peace be upon him, about the procedure of washing the dead body.