(74) باب إِقَامَةِ الصَّفِّ مِنْ تَمَامِ الصَّلاَةِ
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).
Reference: Sahih al-Bukhari 722
In-book reference: Book 10, Hadith 116
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 1, Book 11, Hadith 689