al-Laith bin Sa'd الليث بن سعد · Funerals (Kitab Al-Jana'iz)

The Messenger of Allah (ﷺ) combined two persons from among the martyrs of Uhud (in one garment), and said: Which of the two has learnt the Qur'an more ? When one of them was pointed to him, he advanced him in the grave, saying: I shall be witness to all these (martyrs) on the Day of Judgement. He then ordered them to be buried without being washed.

The Prophet (ﷺ) said: There is no slaughtering (at the grave) in Islam. 'Abd al-Razzaq said: They used to slaughter cows or sheep at grave.