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pilgrimage in Mecca and visit the grave of the Prophet Muhammad peace be upon him in Medina

One of the five pillars of Islam central to Muslim belief, Hajj is the pilgrimage to Mecca that every Muslim must make at least once in their lifetime if they are able; it is the most spiritual event that a Muslim experiences, observing rituals in the most sacred places in the Islamic world. Mecca is the birthplace of the Prophet Muhammad. The sanctuary there with the Ka‘ba is the holiest site in Islam. As such, it is a deeply spiritual destination for Muslims all over the world; it is the heart of Islam.
At the heart of the sanctuary at Mecca lies the Ka’ba, the cube-shaped building that Muslims believe was built by Abraham and his son Ishmael. It was in Mecca that the Prophet Muhammad received the first revelations in the early 7th century.

Therefore the city has long been viewed as a spiritual centre and the heart of Islam. The rituals involved with Hajj have remained unchanged since its beginning, and it continues to be a powerful religious undertaking which draws Muslims together from all over the world, irrespective of nationality or sect.

Even before Islam, Mecca was an important site of pilgrimage for the Arab tribes of north and central Arabia. Although they believed in many deities, they came once a year to worship Allah at Mecca. During this sacred month, violence was forbidden within Mecca and this allowed trade to flourish. As a result, Mecca became an important commercial centre. The revelation of Islam to the Prophet Muhammad (d. 632) restored the ancient religion of the One God to the Arab people and transformed Mecca into the holiest city in the Islamic world.

www.youtube.com/watch?v=kTJZXfaY54w

Dr Zakir Naik about Hajj e badal and Does stonning during hajj hurt shaitan?