Explanation

Abu Hurayrah (may Allah be pleased with him) reported that Khawlah bint Yasār (may Allah be pleased with her) came to the Prophet (may Allah's peace and blessing be upon him) and complained to him that she had only one dress, which would be tainted with menstrual blood during her period. She asked about what she should do. The Prophet (may Allah's peace and blessings be upon him) ordered her to wash it with water when she got purified and then pray in it. She mentioned, however, that traces of blood might remain on the dress after rubbing and washing it well, as related in other narrations of the Hadīth. In response, the Prophet (may Allah's peace and blessings be upon him) explained to her that water is enough for purifying the dress and the remaining trace is harmless, after a person tries his best to wash it.