Musnad Ahmad 1153:319

Muhammad ibn Ubayd narrated to us, Mis'ar narrated to us, from al-Miqdam ibn Shurayh, from his father, from Aisha, who said: "The Messenger of Allah, may Allah bless him and grant him peace, would have a vessel brought to him, and I would drink from it while I was menstruating. Then he would take it and place his mouth where my mouth had been. And I would take a piece of meat with bone (al-'arq) and eat from it, then he would take it and place his mouth where my mouth had been."