It was narrated in a saheeh report from ‘Aa’ishah (may Allaah be pleased with her) that the Prophet (peace and blessings of Allaah be upon him) said: “Allaah does not accept the prayer of a woman of child-bearing age unless (she covers herself) with a khimaar.” (Narrated by Abu Dawood, al-Salaah, 546; classed as saheeh by al-Albaani in Saheeh Sunan Abi Dawood, 596). What this means is that the prayer of an adult woman is not valid (if she does not cover herself), because if something is not accepted, it is not valid.