Musnad Ahmad 30:1634

'Affan narrated to us, Hammad narrated to us, from 'Ali ibn Zayd, from Aws ibn Khalid, from Abu Hurayrah, that the Messenger of Allah (may Allah bless him and grant him peace) said: "People will be gathered in three categories: a category walking, a category riding, and a category upon their faces." They said: "O Messenger of Allah, and how will they walk upon their faces?" He said: "Indeed, He who made them walk upon their feet is capable of making them walk upon their faces. Verily, they will protect themselves with every mound and thorn." 'Affan said: "They will protect themselves with their faces from every mound and thorn."