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كتاب الأذان
10  
Call to Prayers (Adhaan)
(74)
Chapter: The straightening of the rows is amongst those obligatory and good things which make your As-Salat (the prayer) a correct and perfect one.
(74)
باب إِقَامَةِ الصَّفِّ مِنْ تَمَامِ الصَّلاَةِ
Sahih al-Bukhari 722

Narrated Abu Huraira:

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 "Sami`a 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).

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مُحَمَّدٍ، قَالَ حَدَّثَنَا عَبْدُ الرَّزَّاقِ، قَالَ أَخْبَرَنَا مَعْمَرٌ، عَنْ هَمَّامٍ، عَنْ أَبِي هُرَيْرَةَ، عَنِ النَّبِيِّ صلى الله عليه وسلم 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 ‏"‏‏.‏
Reference : Sahih al-Bukhari 722
In-book reference : Book 10, Hadith 116
USC-MSA web (English) reference : Vol. 1, Book 11, Hadith 689
  (deprecated numbering scheme)