Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 853
Abu Huraira reported: one day the Messenger of Allah (may peace be upon him) led the prayer. Then turning (towards his Companions) he said: 0 you, the man, why don't you say your prayer well? Does the observer of prayer not see how he is performing the prayer for he performs it for himself? By Allah, I see behind me as I see In front of me.