Muwatta Malik #378
Shortening the Prayer
The Arabic includes the chain of narration (sanad) and the collector's commentary; the English translation shows the Prophet's words only.
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.
About Muwatta Malik
This hadith is #378 from Muwatta Malik, in the Book of Shortening the Prayer.