Musnad Ahmad 31:242

Hassan narrated to us, Ibn Lahi'ah narrated to us, Darraj narrated to us, from Abu al-Haytham, from Abu Sa'id al-Khudri, from the Messenger of Allah, may Allah bless him and grant him peace, who said: "The seat of the disbeliever in the Fire is a journey of three days, and every molar is like Uhud, and his thigh is like Warqan, and his skin, apart from his flesh and his bones, is forty cubits."