The Command To Perform The Prayer Properly, To Complete It, And To Have Khushu' In It Sahih Muslim 423
Translation
Abu Huraira reported
One day the Messenger of Allah (ﷺ) led the prayer. Then turning (towards his Companions) he said: O you 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.