Yahya related to me from Malik from Nafi that Umar ibn al-Khattabused to order the rows to be straightened, and when they had come tohim and told him that the rows were straight he would say the takbir. (AR)
USC-MSA web (English) reference: Book 9, Hadith 47 (AR)
Arabic reference (AR): Book 9, Hadith 378 (AR)