What is the basis in the Sunnah for the obligation of Hajj?

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The basis in the Sunnah is the Prophet's statement, peace be upon him, 'Islam is built upon five,' which included Hajj. Furthermore, the Prophet, peace be upon him, addressed the people, stating that Allah has obligated Hajj upon them and commanded them to perform it. When asked if it was every year, he remained silent until the third time, then stated that if he had answered affirmatively, it would have become obligatory every year, which they would be unable to fulfill. He then commanded them to follow his instructions to the extent of their ability.