Musnad Ahmad 1219:1

Ya'qub narrated to us, my father narrated to us, from Muhammad ibn Ishaq, who said: Sulayman ibn Suhaym narrated to me, from Umayyah bint Abi al-Salt, from a woman of Banu Ghifar—and he named her to me—she said: I came to the Messenger of Allah, may Allah bless him and grant him peace, among a group of women from Banu Ghifar, and we said to him: "O Messenger of Allah, we have intended to go out with you on this expedition of yours"—and he was marching toward Khaybar—"so that we may treat the wounded and assist the Muslims as much as we are able." He said: "Upon the blessing of Allah." She said: So we went out with him, and I was a young girl. The Messenger of Allah, may Allah bless him and grant him peace, let me ride behind him on the rear part of his saddle. She said: By Allah, the Messenger of Allah, may Allah bless him and grant him peace, did not dismount until morning. He made the camel kneel, and I dismounted from the rear of his saddle, and there was blood from me; it was the first menstruation I had ever experienced. She said: I drew back toward the camel, feeling shy. When the Messenger of Allah, may Allah bless him and grant him peace, saw what was with me and saw the blood, he said: "What is the matter with you? Perhaps you have begun to menstruate?" She said: I said, "Yes." He said: "Then rectify yourself, take a vessel of water, put salt in it, then wash the blood that touched the saddle, then return to your mount." She said: When the Messenger of Allah, may Allah bless him and grant him peace, conquered Khaybar, he gave us a portion of the spoils, and he took this necklace that you see on my neck and gave it to me, and he placed it with his own hand around my neck. By Allah, it shall never leave me. He said: It remained on her neck until she died, then she willed that it be buried with her. She would not become pure from menstruation without putting salt in her purification water, and she willed that it be put in her washing when she died.