Musnad Abu Ya'la al-Mawsili 42:415

From Abu Sa'id, from the Messenger of Allah (may Allah bless him and grant him peace), he said: The seat of the disbeliever in the Fire is three days' journey. Every molar of his is like Uhud, and his thigh is like Warqan. And the thickness of his skin is forty cubits, apart from his flesh and his bones.