Musnad Ahmad 31:399

'Affan narrated to us, Hammad ibn Salamah narrated to us, from al-Jurayri, from Abu Nadrah, from Abu Sa'id al-Khudri, that the Messenger of Allah (peace and blessings of Allah be upon him) asked Ibn Sa'id about the soil of Paradise. He said: "A white flour-like substance, pure musk." He said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "He has spoken the truth."