The Messenger of Allah ﷺ combined Hajj and Umrah together, and the people combined them along with him.
أن رسول الله ﷺ قرن بين الحج والعمرة، وقرن القوم معه.
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Ibn Hibban (3931) narrated it from the hadith of Ashath, that Hasan told them, from Anas bin Malik, and he related it. Nasai (2662, 2756) also narrated it by this route, except that he said, "and he entered the state of ihram for Hajj and Umrah when he prayed Dhuhr." Its chain of transmission is sahih. Hasan was a mudallis and he narrated using an "ana" formula, but his hearing from Anas has been established, as Ahmad stated. Ibn Hibban's inclusion of it is evidence, in his view, of an explicit statement of direct transmission, as he made clear in the introduction to his Sahih. The other narration from Anas confirms the soundness of this hadith.