Musannaf Ibn Abi Shayba 33:727

Amr ibn Sa‘d Abu Dawud narrated to us, from Sufyan, from al-A‘mash, from ‘Abd al-Rahman ibn Abi Ziyad, from ‘Abd al-Rahman ibn Abi Layla, who said: The soul is in the hand of an angel who walks with it. When he enters his grave, he places it into it.