Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.013.

If the clothes or body are contaminated with impurities, it is obligatory to wash them with water until they are cleansed of the impurities. This is especially the case if the impurity is visible, such as blood. If there are some stains that remain after washing which would be extremely difficult to remove, they can be overlooked. If the impurity is not visible, such as urine, it is sufficient to wash it one time. ‘Asma bint Abu Bakr related that a woman came to the Prophet, upon whom be peace, and said, “Our clothes are contaminated with menstrual blood. What should we do about this?” He said, “Scrape it, rub it with water, pour water over it and then pray in it.” (This is related by al-Bukhari and Muslim) If impurities get on the lower portion of a woman’s dress, it is purified by dust as she trails along. A woman said to Umm Salamah, “I have a long dress that drags on the ground, even when I walk through places that contain filth. What should I do about it?” Umm Salamah answered her, “The Messenger of Allah said, ‘What comes after it purifies it.”‘ This is related by Ahmad and Abu Dawud.