Abu Dawud Book 1, Hadith Number 313.
Chapter : A person who is sexually defiled is permitted to eat anything before washing.
Narated By Woman of Banu Ghifar : Umayyah, daughter of AbusSalt, quoted a certain woman of Banu Ghifar, whose name was mentioned to me, as saying: The Apostle of Allah (PBUH) made me ride behind him on the rear of the camel saddle. By Allah, the Apostle of Allah (PBUH) got down in the morning. He made his camel kneel down and I came down from the back of his saddle. There was a mark of blood on it (saddle) and that was the first menstruation that I had. I stuck to the camel and felt ashamed.
When the Apostle of Allah (PBUH) saw what had happened to me and saw the blood, he said: Perhaps you are menstruating.
I said: Yes. He then said: Set yourself right (i.e. tie some cloth to prevent bleeding), then take a vessel of water and put some salt in it, and then wash the blood from the back of the saddle, and then return to your mount. When the Apostle of Allah (PBUH) conquered Khaybar, he gave us a portion of the booty. Whenever the woman became purified from her menses, she would put salt in water. And when she died, she left a will to put salt in the water for washing her (after death).