Sayyidah Aishah (may Allah be pleased with her) said: In the Farewell Hajj I entered ihram for Umrah with the Messenger of Allah (peace and blessings be upon him). Then she narrated the hadith about her menstruation: I said: O Messenger of Allah! This is the day of Arafah and I have still not become pure, and I had intended to benefit from Umrah (tamattu'). The Messenger of Allah (peace and blessings be upon him) said: "Undo your hair, comb it, enter ihram for Hajj, and stop (the rites of) your Umrah." She said: So I did that.
(b) Her bath was for the talbiyah of Hajj and was a sunnah bath; the order in it to undo her hair and comb it was recommended (mustahabb), just as the order to Asma bint Umays (may Allah be pleased with her) to take the ritual bath during postnatal bleeding (nifas) was recommended.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 862