Musannaf Ibn Abi Shayba 3:1800

Waki’ narrated to us, from ‘Imran bin Hudayr, from Abu Mijlaz, who said: The prayer was called, and the rows were straightened. A man hurried to ‘Umar and spoke to him, and they remained standing for a long time until they [the people] sat upon the ground, while the people were in rows.