Musnad Ahmad 30:3434

Yazid narrated to us, Ibn Abi Dhi'b informed us, from 'Ajlan, from Abu Hurairah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "By the One in whose hand is the soul of Muhammad, I truly see what is behind me just as I see what is in front of me. So straighten your rows, and perfect your bowing and your prostration."