Musnad Abu Ya'la al-Mawsili 148:28

Ubayd Allah ibn Umar al-Qawariri narrated to us, Yazid ibn Harun narrated to us, Isma'il ibn Abi Khalid narrated to us, from Abu Dawud, from Anas ibn 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."