Musnad Ahmad 25:490

Ubaydah ibn Humayd narrated to us, from Yazid ibn Abi Ziyad, from Mujahid, from Ibn Abbas, that the Messenger of Allah (ﷺ) said to his companions: "Make it an Umrah, for if I had known in the beginning what I have come to know in the end, I would have commanded you to do so. And let him who does not have a sacrificial animal with him exit the state of Ihram." And the Messenger of Allah (ﷺ) had a sacrificial animal with him. He said: And the Messenger of Allah (ﷺ) said: "The Umrah has entered into the Hajj until the Day of Resurrection," and he interlaced his fingers.