Abu Dawud Book 2, Hadith Number 1409.
Chapter : What should one say when one prostrates.
Narated By ‘Aisha, Ummul Mu’minin : The Apostle of Allah (PBUH) prostrated himself at night when reciting the Qur’an. He said repeatedly: My face prostrates itself to Him Who created it and brought forth its hearing and seeing by His might and power.