Al-Matalib al-'Aliyah 2591 — Etiquette of sleep

It is narrated from a member of the household of the Mother of the Believers, Sayyidah Umm Salamah (may Allah be pleased with her), that he said: "The bed of the Messenger of Allah (peace and blessings be upon him) was (in its size and shape) roughly like the way a person is placed in his grave (in the lahd), and the mosque (its direction or its wall) was at the side of his head."
This report is mursal hasan (a good report with a missing Companion link).