And the evidence that if the one praying looks at the women behind him, that action does not invalidate his prayer.

Sayyiduna Ibn Abbas (may Allah be pleased with them both) narrates that among the most beautiful people, there was a very attractive and beautiful woman who used to pray behind the Messenger of Allah (peace and blessings be upon him). So some people would pray in the front rows so that they would not see her, and some people would stay behind so that they would be in the back rows. Then, when they would bow (in ruku‘), they would look at her from under their armpits. So Allah Almighty revealed this verse about this matter: «‏‏‏‏وَلَقَدْ عَلِمْنَا الْمُسْتَقْدِمِينَ مِنكُمْ وَلَقَدْ عَلِمْنَا الْمُسْتَأْخِرِينَ» ‏‏‏‏ [ سورة الحجر: 24 ] "And surely We know those among you who go forward and We know those who lag behind."

The Imam, through his teacher Abu Musa’s chain of narration, reports a narration with the same meaning as mentioned above.