Musannaf Ibn Abi Shayba 36:495

'Abd al-A'la narrated to us, from Ma'mar, from al-Zuhri, from Sa'id ibn al-Musayyib, that those who undertook the burial of the Messenger of Allah, may Allah bless him and grant him peace, and his interment were four people, excluding the [rest of the] people: 'Ali, al-'Abbas, al-Fadl, and Salih, the freed slave of the Prophet, may Allah bless him and grant him peace. They dug a *lahd* (niche) for him and set up the mud bricks over him upright.