The Kaaba is the most sacred spot on earth for Muslims. The Kaaba is located in Mecca (or Makkah), in the Kingdom of Saudi Arabia. It is the most sacred city for Muslims for having Al-Masjid Al-Haram (or the Grand Mosque) that surrounds the Kaaba.
The Qur'an is the most sacred book of the Muslims.
Jerusalem. However, Jerusalem is a sacred city for also both Christians and Muslims.
No one specific color is ''sacred'', however the color most often associated with Muslims is green, because the Ottomans, who used it in their flag, were primarily Muslim.
Makkah (or Mecca) in the Kingdom of Saudi Arabia
The Qur'an is the most sacred book of the Muslims.
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The most sacred text of the Muslims is the holy Qur'an, that is the word of Almighty Allah revealed to the last Prophet of Islam (SAW).The collections of the sayings of the holy Prophet (SAW) are also sacred books of the Muslims.
Mecca, in Saudi Arabia, is the most sacred city in Islam, to which millions of Muslims travel every year on Pilgrimage.
Muslims don't worship any building. Even, the Kaaba (that is the most sacred Islamic building) is not worshiped by Muslims but respected and honored by Muslims. If you wish to read about places where Muslims worship (both houses of worship and sacred sites) read the Related Question below.
Mecca (or Makkah) is forbidden for non Muslims to visit or stay because it is the Muslims most sacred and most holiest city. It is the same way Vatican is forbidden for Muslims to stay in.
Because Kaba is the holiest and most sacred and honored building for Muslims.