Musnad Ahmad 31:10

Abu Amir Abd al-Malik ibn Amr narrated to us, Zuhayr—meaning Ibn Muhammad—narrated to us, from Abdullah ibn Muhammad ibn Aqil, from Said ibn al-Musayyib, from Abu Said al-Khudri, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Shall I not guide you to that by which Allah effaces sins and increases good deeds?" They said: "Yes, O Messenger of Allah." He said: "Performing wudu thoroughly despite difficulties, increasing steps toward these mosques, and waiting for the prayer after the prayer. There is no man among you who leaves his house in a state of purity, then prays the prayer with the Muslims, then sits in the gathering waiting for the other prayer, except that the angels say: 'O Allah, forgive him; O Allah, have mercy upon him.' So when you stand for prayer, straighten your rows, set them up, and close the gaps, for I see you from behind my back. When your Imam says 'Allahu Akbar,' then say 'Allahu Akbar.' When he bows, then bow. And when he says 'Sami Allahu liman hamidah,' then say 'Allahumma Rabbana lakal-hamd.' Indeed, the best of the rows for men are the front ones, and the worst of them are the back ones. The best of the rows for women are the back ones, and the worst of them are the front ones. O assembly of women, when the men prostrate, lower your gazes so that you do not see the private parts of the men due to the tightness of their lower garments."