Musnad Ahmad 30:1226

Rab'i ibn Ibrahim narrated to us, Abd al-Rahman ibn Ishaq narrated to us, from Sa'id ibn Abi Sa'id, from Abu Hurayrah, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "The molar of the disbeliever on the Day of Resurrection will be like Uhud, and the thickness of his skin will be seventy cubits, and his thigh will be like Warqan, and his seat in the Fire will be like the distance between me and al-Rabadhah."