Musnad Abu Ya'la al-Mawsili 210:25

Zuhayr narrated to us, Abd al-Rahman narrated to us, from Malik, from Nafi, from Ibn Umar, from Hafsa, that she said to the Prophet (peace be upon him): "What is the matter with you that you did not exit your Umrah?" He said: "I have matted my hair and garlanded my sacrificial animal, so I will not exit [the state of Ihram] until I slaughter."