Musannaf Ibn Abi Shayba 3:1160

Abu Mu'awiyah narrated to us, from 'Asim, from Abu 'Uthman, who said: I have not seen anyone who was more strict in ensuring the alignment of the rows than 'Umar. He would face the Qiblah, and when we said [to ourselves] that he had [already] said the Takbir, he would turn and look at the shoulders and the feet. And he used to send men who would drive the people [forward] until they joined them to the rows.