Yahya related to me from Malik from Hisham ibn Urwa that hisfather said, "There were two men in Madina, one of whom dug graveswith a niche in the side wall for the body, and the other who did not,and they said, 'Whichever one comes first can do the job,' and the onewho dug graves with a niche came first and dug the Messenger ofAllah's grave, may Allah bless him and grant him peace."
USC-MSA web (English) reference: Book 16, Hadith 28
Arabic reference: Book 16, Hadith 550