The Prophet ﷺ said, "The Imam is (appointed) to be followed. So do not differ from him, bow when he bows, and say, "Rabbana-lakal hamd" if he says "Samiʿa l-lahu liman hamidah"; and if he prostrates, prostrate (after him), and if he prays sitting, pray sitting all together, and straighten the rows for the prayer, as the straightening of the rows is amongst those things which make your prayer a correct and perfect one. (See Hadith No. 657).  

عَنِ النَّبِيِّ ﷺ أَنَّهُ قَالَ إِنَّمَا جُعِلَ الإِمَامُ لِيُؤْتَمَّ بِهِ فَلاَ تَخْتَلِفُوا عَلَيْهِ فَإِذَا رَكَعَ فَارْكَعُوا وَإِذَا قَالَ سَمِعَ اللَّهُ لِمَنْ حَمِدَهُ فَقُولُوا رَبَّنَا لَكَ الْحَمْدُ وَإِذَا سَجَدَ فَاسْجُدُوا وَإِذَا صَلَّى جَالِسًا فَصَلُّوا جُلُوسًا أَجْمَعُونَ وَأَقِيمُوا الصَّفَّ فِي الصَّلاَةِ فَإِنَّ إِقَامَةَ الصَّفِّ مِنْ حُسْنِ الصَّلاَةِ  

The Prophet ﷺ said, "Straighten your rows as the straightening of rows is essential for a perfect and correct prayer. "  

عَنِ النَّبِيِّ ﷺ قَالَ سَوُّوا صُفُوفَكُمْ فَإِنَّ تَسْوِيَةَ الصُّفُوفِ مِنْ إِقَامَةِ الصَّلاَةِ