Muwatta Imam Malik Book of Shortening the Prayer, Hadith 47

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 .