Musnad Ahmad 30:3160

'Abd al-Rahman narrated to us, from Zuhayr—meaning Ibn Muhammad al-Khurasani—and Abu 'Amir, who said: Zuhayr narrated to us, from al-'Ala', from his father, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "Perfect the straightening of the rows in prayer. The best of the men's rows in prayer is the first, and the worst of them is the last. And the best of the women's rows in prayer is the last, and the worst of them is the first."