"The Messenger of Allah ﷺ prayed one day then left and said: '0 so-and-so, why don't you improve your prayer? Shouldn't the one who is praying reflection how he prays it for himself? I can see behind me just as I can see in front of me."'  

صَلَّى رَسُولُ اللَّهِ ﷺ يَوْمًا ثُمَّ انْصَرَفَ فَقَالَ يَا فُلاَنُ أَلاَ تُحَسِّنُ صَلاَتَكَ أَلاَ يَنْظُرُ الْمُصَلِّي كَيْفَ يُصَلِّي لِنَفْسِهِ إِنِّي أُبْصِرُ مِنْ وَرَائِي كَمَا أُبْصِرُ بَيْنَ يَدَىَّ  

"The Messenger of Allah ﷺ prayed one day then left and said: '0 so-and-so, why don't you improve your prayer? Shouldn't the one who is praying reflection how he prays it for himself? I can see behind me just as I can see in front of me."' (Using translation from Nasāʾī 872)  

صَلَّى رَسُولُ اللهِ ﷺ يَوْمًا ثُمَّ انْصَرَفَ فَقَالَ «يَا فُلَانُ أَلَا تُحْسِنُ صَلَاتَكَ أَلَا يَنْظُرُ الْمُصَلِّي كَيْفَ يُصَلِّي لِنَفْسِهِ فَإِنِّي أُبْصِرُ مِنْ وَرَائِي كَمَا أُبْصِرُ مِنْ بَيْنَ يَدَيَّ»  

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. (Using translation from Muslim 423)   

صَلَّى رَسُولُ اللهِ ﷺ يَوْمًا ثُمَّ انْصَرَفَ فَقَالَ يَا فُلَانُ أَلَا تُحْسِنُ صَلَاتَكَ؟ أَلَا يَنْظُرُ الْمُصَلِّي إِذَا صَلَّى كَيْفَ يُصَلِّي فَإِنَّمَا يُصَلِّي لِنَفْسِهِ إِنِّي وَاللهِ لَأُبْصِرُ مَنْ وَرَائِي كَمَا أُبْصِرُ مَنْ بَيْنِ يَدِيَّ  

"يَا فُلَانُ: أَلَا تُحْسِنُ صَلَاتَكَ؟ أَلَا يَنْظُرُ الْمُصَلِّى إِذَا صَلَّى كَيْفَ يُصَلىِّ، فَإِنَّمَا يُصَلِّى لِنَفْسِهِ، إِنِّى وَالله لأُبْصِرُ مِنْ وَرَائِى كَمَا أُبْصِرُ مِنْ بَيْن يَدَىَّ".