A woman of Banu Ghifar (may Allah be pleased with her) said: I came to the Prophet (peace and blessings be upon him) together with the women of my tribe. We said: O Messenger of Allah! We too want to go with you to Khaybar so that we can treat the wounded and help the Muslims as far as we are able. We set out with him (peace and blessings be upon him), and I was a young girl. The Prophet (peace and blessings be upon him) seated me behind him on the bag of his saddle. In the morning he (peace and blessings be upon him) halted and dismounted, and as I was getting down from the mount I noticed blood on the baggage; it was my first menstruation. I stayed sitting on the mount, feeling ashamed. When the Prophet (peace and blessings be upon him) saw the blood, he said: "Perhaps you have started menstruating?" I said: Yes. He (peace and blessings be upon him) said: "Tidy yourself up, then take a vessel of water and mix salt into it. Wash the saddlebag with it and take a ritual bath yourself, then return to your mount."
So whenever she menstruated, she would purify herself with water and salt, and she left instructions that, after her death, her washing also be done with salt mixed in water.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4111