Mishkat al-Masabih 1086
Anas said:
The iqama * had been uttered and God’s Messenger faced us and said, “Make your rows straight and stand close together, for l can see you behind my back.” 

* A repetition of the words of the call to prayer after the worshippers have taken their places with the addition of a statement that the time of prayer has come.

Bukhari transmitted it. In the version given by Bukhari and Muslim he said, “Make the rows perfect, for I can see you behind my back."
وَعَن أنس قَالَ: أُقِيمَتِ الصَّلَاةُ فَأَقْبَلَ عَلَيْنَا رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ بِوَجْهِهِ فَقَالَ: «أَقِيمُوا صُفُوفَكُمْ وَتَرَاصُّوا فَإِنِّي أَرَاكُمْ مِنْ وَرَاءِ ظَهْرِي» . رَوَاهُ الْبُخَارِيُّ. وَفِي الْمُتَّفَقِ عَلَيْهِ قَالَ: «أَتِمُّوا الصُّفُوف فَإِنِّي أَرَاكُم من وَرَاء ظَهْري»