Musnad Ahmad 32:560

Aswad ibn Amir or Hasan ibn Musa narrated to us, Zuhayr narrated to us, from Abu Ishaq, from Abu Asma al-Sayqal, from Anas ibn Malik, who said: We set out shouting the Talbiyah for Hajj. When we arrived in Makkah, the Messenger of Allah (peace and blessings of Allah be upon him) commanded us to change it into an Umrah. He said: "If I had known in the beginning what I know now, I would have made it an Umrah, but I have brought the sacrificial animal and have combined Hajj and Umrah."