Sayyiduna Ibn Abbas (may Allah be pleased with them both) narrates that a man from among those who had set out (for Hajj) would come to the Messenger of Allah (peace and blessings be upon him), mentioning only Hajj, and the Messenger of Allah (peace and blessings be upon him) would say: "Go and, after performing tawaf, leave ihram by making it Umrah." The man would say: O Messenger of Allah, I set out intending nothing but Hajj. He (peace and blessings be upon him) would say: "This is not Hajj; rather, it is Umrah."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12105