Musnad Ahmad 1155:15

Ya'qub ibn Ibrahim narrated to us, my father narrated to us, from Ibn Ishaq, he said: Nafi' narrated to me, from Abdullah ibn Umar, from Hafsa bint Umar, she said: When the Messenger of Allah, peace and blessings of Allah be upon him, commanded his wives to exit ihram for 'Umrah, they said: "What prevents you, O Messenger of Allah, from exiting ihram with us?" He said: "I have brought the sacrificial animal (hady) and I have matted my hair (labadtu), so I will not exit ihram until I slaughter my sacrificial animal." And Ya'qub said in the Book of Hajj: "Slaughter my sacrificial offering."