Musnad Abu Ya'la al-Mawsili 210:19

Khalf ibn Hisham narrated to us, Ali ibn Mushir narrated to us, from Ubayd Allah, from Nafi, from Ibn Umar, from Hafsa, who said: When the Messenger of Allah (peace be upon him) commanded us to exit [the state of Ihram] for an Umrah, I said: "O Messenger of Allah, what prevents you from exiting [the state of Ihram] with us?" He said: "I have matted my hair and garlanded my sacrificial animal, so I will not exit [the state of Ihram] until I slaughter."