Fiqh-us-Sunnah Volume 4, Funerals and Dhikr, Fiqh 4.026C.

The majority of jurists are of the opinion that washing the body of a dead Muslim is a fard kifayah or a collective obligation. If some people attend to it, it is done on behalf of all, as commanded by Allah’s Messenger, peace be upon him, and practiced by the Muslim community.