Hajj is the fifth pillar of Islam. It is obligatory for every Muslim to perform Hajj once in his or her lifetime unless there is no financial or physical ability. Allah (SWT) has said, "Say: "Allah spoke the truth: follow the religion of Abraham, the sane in faith; he was not of the Pagans."
The first house (of worship) appointed for people was that at Bakkah, full of blessings and guidance for all kinds of beings. In it are signs manifest; (for example, the Station of Abraham; whoever enters it attains security; pilgrimage thereto is a duty people owe to Allah, those who can afford the journey; but if any deny faith, Allah stands not in need of any of His creatures." (2:96-97) "And complete Hajj and 'Umrah in the service of Allah." (2:196)
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"For Hajj, the months are well known. If anyone undertakes that duty therein, let there be no obscenity, nor wickedness, nor wrangling in the Hajj, and whatever good you do, (be sure) Allah knows it. And take a provision with you for the journey, but the best of provisions is right conduct. So, fear me, O you that are wise!" (2:198)
The Prophet (S) said, "O people! Perform Hajj, for Allah has made it obligatory on you." Hearing this, a man said, "Is it every year, O Messenger of Allah?" The Prophet (S) kept silent until the man repeated the question three times and then said, "If I say yes, it will become compulsory, and you won't be able to do it." [Muslim and Ahmad].
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Ibn 'Abbas (r) said, "Allah's Messenger (S) addressed us, saying, "O people! Allah has prescribed Hajj on you". Al-Aqra'a ibn Haabis then stood up and said, "Is it for every year, O Messenger of Allah?" The Prophet (S) replied, "If I say yes, it would become obligatory, and if it's obligatory to perform Hajj annually, you certainly won't be able to do it. Hajj should be performed only once in one's lifetime, and whoever does more, it will be considered supererogatory." [Related by An-Nassa'i, Abu Dawud, Ibn Maajah, and Ahmad].
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"Islam was built on five pillars: the Shahadah, a declaration that there is no one worthy of worship except Allah and that Muhammad is the Messenger of Allah, performing prayer, giving Zakaat, performing Hajj, and fasting Ramadan." [Agreed upon]