Musnad Ahmad 31:18

Rawh narrated to us, Hammad narrated to us, al-Jurayri narrated to us, from Abu Nadra, from Abu Sa’id al-Khudri, that the Prophet, peace and blessings of Allah be upon him, asked Ibn Sayyad about the soil of Paradise. He said: "A white mound of pure musk." The Messenger of Allah, peace and blessings of Allah be upon him, said: "He has spoken the truth."