Musnad Ahmad 30:1136

Hashim ibn al-Qasim narrated to us, Ibn Abi Dhi'b narrated to us, from 'Ajlan, from Abu Hurairah, from the Messenger of Allah, may Allah bless him and grant him peace, who said: "By the One in whose hand is my soul, I truly look at what is behind me just as I look at what is in front of me. So straighten your rows, and perfect your bowing and your prostration."