Sayyiduna Ibn Abbas (may Allah be pleased with them both) narrates that the Prophet (peace and blessings be upon him) said: "Jibril (peace be upon him) came to me when three nights of Dhu al-Qa'dah remained and said: 'Umrah has entered into Hajj until the Day of Resurrection.'" On that occasion the Messenger of Allah (peace and blessings be upon him) said: "Had I known beforehand what I came to know later, I would not have brought the sacrificial animal (hady) with me."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12905