Allah said,

﴿وَللَّهِ عَلَى النَّاسِ حِجُّ الْبَيْتِ مَنِ اسْتَطَـعَ إِلَيْهِ سَبِيلاً﴾

(And Hajj to the House is a duty that mankind owes to Allah, for those who are able to undertake the journey) ﴿3:97﴾.

This Ayah established the obligation of performing Hajj. There are many Hadiths that mention it as one of the pillars and fundamentals of Islam, and this is agreed upon by the Muslims. According to texts and the consensus of the scholars, it is only obligatory for the adult Muslim to perform it once during his lifetime. Imam Ahmad recorded that Abu Hurayrah said that the Messenger of Allah once gave a speech in which he said,

«أَيُّهَا النَّاسُ قَدْ فُرِضَ عَلَيْكُمُ الْحَجُّ فَحُجُّوا»

(O people! Hajj has been enjoined on you, therefore, perform Hajj.)

A man asked, "Is it every year, O Allah's Messenger'' The Prophet remained silent until the man repeated the question three times and he then said,

«لَوْ قُلْتُ: نَعَمْ لَوَجَبَتْ وَلَمَا اسْتَطَعْتُم»

(Had I said yes, it would have become an obligation and you would not have been able to fulfill it.) He said next,

«ذَرُونِي مَا تَرَكْتُكُمْ فَإِنَّمَا هَلَكَ مَنْ كَانَ قَبْلَكُمْ بِكَثْرَةِ سُؤَالِهِمْ وَاخْتِلَافِهِمْ عَلى أَنْبِيَائِهِمْ، وَإِذَا أَمَرْتُكُمْ بِشَيْءٍ فَأْتُوا مِنْهُ مَا اسْتَطَعْتُمْ، وَإِذَا نَهَيْتُكُمْ عَنْ شَيْءٍ فَدَعُوه»

(Leave me as I leave you, those before you were destroyed because of their many questions and disputing with their Prophets. If I command you with something, perform it as much as you can. If I forbid something for you, then refrain from it.) Muslim recorded similarly.