Musannaf Ibn Abi Shayba 34:185

Abu Bakr al-Hanafi narrated to us, from Kathir ibn Zayd al-Madani, from al-Muttalib ibn Abdullah ibn Hantab, who said: When Uthman ibn Maz'un died, the Messenger of Allah (peace be upon him) buried him in al-Baqi'. He was the first to be buried in it. Then he said to a man who was with him: "Go to that rock and bring it to me, so that I may place it at his grave, that I may recognize him by it, and whoever dies from among our family, we shall bury him beside him."