Abu al-Bada' says: I entered, with Mus'ab bin al-Zubayr (may Allah have mercy on him), the house in which the grave of the Prophet (peace and blessings be upon him) is. I saw their graves like flat, spread-out strips.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 6759