All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and messenger.
Ibn ‘Umar narrated: "During the lifetime of the Prophet men and women performed ablution together." [Al-Bukhari]
The statements of the scholars varied concerning the meaning of this Hadeeth. Some of them said that men and women would perform ablution in one place, but men were separated from women. Other scholars said that men would perform ablution and then leave, and once they left, women would come and perform ablution. Ibn Hajar did not agree with both interpretations. He suggested that this used to take place before Hijab was ordained. He [Ibn Hajar] said: “It is more appropriate to say that there was nothing wrong with men gathering with women before the verse of Hijab was revealed. However, after Hijab was ordained, gathering became lawful only with wives and female Mahaarim (non-Marriageable female relatives).”
What further supports the prohibition of men and non-Mahram women performing ablution together is that Abdur-Razzaaq reported about Abu Salaamah Al-Habeebi; he said: “I saw ‘Umar ibn Al-Khattaab come to a basin and found men and women performing ablution together, and he beat them with a whip, and then he said to the owner of the basin: ‘Make a separate basin for men and another for women.’” [End of quote]
Allaah Knows best.