Musnad Abu Ya'la al-Mawsili 148:29

Zuhayr narrated to us, Yazid narrated to us, Isma'il bin Abi Khalid informed us, from Abu Dawud al-A'ma, from Anas bin Malik, who said: The Messenger of Allah (may Allah bless him and grant him peace) was asked: "How will the people be gathered upon their faces?" He said: "Indeed, He who made them walk upon their feet is capable of making them walk upon their faces."