Musnad Ahmad 31:205

Yunus ibn Muhammad narrated to us, Hammad narrated to us, from al-Jurayri, from Abu Nadra, 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: "It is white flour of musk." He said: Then the Messenger of Allah (peace and blessings of Allah be upon him) said: "He has spoken the truth."