The Prophet ﷺ said, "Bury them (i.e. martyrs) with their blood." (that was) On the day of the Battle of Uhud. He did not get them washed.  

قَالَ النَّبِيُّ ﷺ ادْفِنُوهُمْ فِي دِمَائِهِمْ يَعْنِي يَوْمَ أُحُدٍ وَلَمْ يُغَسِّلْهُمْ