Musnad Abu Ya'la al-Mawsili 148:95

Abu Khaythama narrated to us, al-Hasan ibn Musa narrated to us, Zuhayr narrated to us, from Abu Ishaq, from Abu Asma al-Saqil, from Anas ibn Malik, who said: We went out shouting for Hajj. When we arrived in Makkah, the Messenger of Allah (may Allah bless him and grant him peace) commanded us to make it an Umrah. He said: "If I had known in the beginning what I know now, I would have made it an Umrah, but I brought the sacrificial animal and combined the Hajj and the Umrah."