كتاب 9
9  
Book 9
(9)
Chapter 9
(9)
باب 9
Muwatta Malik 378
Yahya related to me from Malik from Nafi that Umar ibn al-Khattab used to order the rows to be straightened, and when they had come to him and told him that the rows were straight he would say the takbir.
حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ عُمَرَ بْنَ الْخَطَّابِ، كَانَ يَأْمُرُ بِتَسْوِيَةِ الصُّفُوفِ فَإِذَا جَاءُوهُ فَأَخْبَرُوهُ أَنْ قَدِ اسْتَوَتْ كَبَّرَ ‏.‏
Reference : Muwatta Malik 378
In-book reference : Book 9, Hadith 34556
USC-MSA web (English) reference : Book 7, Hadith 378
  (deprecated numbering scheme)