مُوَطَّأُ مَالِكٍ
Muwatta Malik 378
Shortening the Prayer
حَدَّثَنِي يَحْيَى، عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ عُمَرَ بْنَ الْخَطَّابِ، كَانَ يَأْمُرُ بِتَسْوِيَةِ الصُّفُوفِ فَإِذَا جَاءُوهُ فَأَخْبَرُوهُ أَنْ قَدِ اسْتَوَتْ كَبَّرَ .
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.
Arabic/English book reference: Book 9, Hadith 47