It was narrated that Anas said:

"The Messenger of Allah ﷺ turned to face us when he stood up to pray, before he said Takbir, and said: 'Make your rows straight and fill the gaps, for I can see you from behind my back."'

أَنْبَأَنَا عَلِيُّ بْنُ حُجْرٍ، قَالَ أَنْبَأَنَا إِسْمَاعِيلُ، عَنْ حُمَيْدٍ، عَنْ أَنَسٍ، قَالَ أَقْبَلَ عَلَيْنَا رَسُولُ اللَّهِ ﷺ بِوَجْهِهِ حِينَ قَامَ إِلَى الصَّلاَةِ قَبْلَ أَنْ يُكَبِّرَ فَقَالَ " أَقِيمُوا صُفُوفَكُمْ وَتَرَاصُّوا فَإِنِّي أَرَاكُمْ مِنْ وَرَاءِ ظَهْرِي " .