Source: The Great Interpretation of Dreams (Ibn Sirin)
Washing a dead person in a dream indicates the repentance of a corrupt man at the hands of the dreamer. Whoever sees himself washing a dead person will be saved from worry and grief.
Source: Perfuming Humanity in the Expression of Dreams (Al-Nabulsi)
Washing a dead person indicates paying off their debt or executing their will. Whoever sees himself being washed while alive, it indicates the removal of his worry and his salvation.
Source: Signs in the Science of Expressions (Ibn Shaheen)
Washing the dead is interpreted as goodness. Whoever sees himself washing a dead person will attain goodness.