Musnad Abu Ya'la al-Mawsili 161:10

Ahmad ibn Isa al-Masri narrated to us, Ibn Wahb narrated to us, Amr ibn al-Harith informed me, from Umarah ibn Ghaziya, from Sumayy, that he heard Abu Salih, from Abu Hurayrah, that the Messenger of Allah (may Allah bless him and grant him peace) said: "The closest a servant is to his Lord is while he is prostrating, so increase the supplication."