From Sahih Bukhari (Hadith 1353):
Narrated Jabir bin `Abdullah:The Prophet (ﷺ) collected every two martyrs of Uhud (in one grave) and then he would ask, "Which of them knew the Qur'an more?" And if one of them was pointed out for him as having more knowledge, he would put him first in the Lahd. The Prophet (ﷺ) said, "I will be a witness on these on the Day of Resurrection." Then he ordered them to be buried with their blood on their bodies and he did not have them washed
Reference: Bukhari 1353