Musnad Ahmad 8:162

Abu Sa’id, the freed slave of Banu Hashim, narrated to us, ‘Abdullah ibn Ja’far narrated to us, Isma’il ibn Muhammad narrated to us, from ‘Amir ibn Sa’d, that Sa’d said during his illness: "When I die, then dig a *lahd* (a niche in the side of the grave) for me, and do just as was done for the Messenger of Allah, may Allah bless him and grant him peace."