Musnad Ahmad 1155:10

'Abd al-Rahman ibn Mahdi narrated to us, from Malik, from Nafi', from Ibn 'Umar, from Hafsah, that she said to the Prophet (peace and blessings of Allah be upon him): "What is the matter with you that you have not exited 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."