Musnad Ahmad 30:1526

Hassan ibn Musa and ‘Affan narrated to us, they said: Hammad ibn Salamah narrated to us, from ‘Ali ibn Zayd, from Aws, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "People will be gathered on the Day of Resurrection in three categories: a category walking, a category riding, and a category upon their faces." They said: "O Messenger of Allah, how will they walk upon their faces?" And ‘Affan said: "They will walk." 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 their faces from every mound and thorn."