It is narrated from Sayyiduna Jabir (may Allah be pleased with him) that a lahd (niche grave) was dug for the Prophet (peace and blessings be upon him) and unbaked bricks were set up over him. Then he mentioned the rest of the hadith and said that the grave of the Prophet (peace and blessings be upon him) was raised from the ground by about a hand-span. This is how I found it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنائز / 6736