Musnad Ahmad 32:765

Ibn Numayr narrated to us, Isma'il ibn 'Umar narrated to us, from Nufay', who said: I heard Anas ibn Malik saying: It was said: "O Messenger of Allah, 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."