Musannaf Ibn Abi Shayba 6:916

Yahya ibn Sa'id narrated to us, from Thawr, who said: 'Asim ibn Hushayb (and others from the people of al-Sham) narrated to us, from Abu al-Darda', who said: "From the completion of the reward of the funeral is that one casts [soil] into the grave."