Imam al-Shafi'i (may Allah have mercy on him) said regarding the words of Sayyidah Hafsah (may Allah be pleased with her) to the Prophet (peace and blessings be upon him), "and you have not come out of ihram from your Umrah": she meant, from your ihram which you began, while they were (all) upon one intention, and Allah knows best. So he (peace and blessings be upon him) said: "I have matted my hair and garlanded my sacrificial animal, so I will not come out of ihram until I sacrifice." Meaning, and Allah knows best, until the pilgrim comes out of ihram, because the decision had been revealed to him that whoever had a sacrificial animal with him should make his ihram a Hajj.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 8845