Grave worshiper beware:

Grave worshiper beware:
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Praise be to Allaah.

The first type is praying to the occupant of the grave. This is major shirk which puts a person beyond the pale of Islam, because prayer is an act of worship, and it is not permissible to do any act of worship to anyone other than Allaah. Allaah says (interpretation of the meaning):

“Worship Allaah and join none with Him (in worship)”
[al-Nisa’ 4:36]

“Verily, Allaah forgives not (the sin of) setting up partners (in worship) with Him, but He forgives whom He wills, sins other than that, and whoever sets up partners in worship with Allaah, has indeed strayed far away”
[al-Nisa’ 4:116]
(for details, check: http://islamqa.info/en/13490)

It is bid'ah (innovation), which is visiting graves for the purpose of calling upon their occupants, seeking their help, offering sacrifices to them and making vows to them. This is forbidden and is a major form of Shirk (shirk akbar). Connected to this is the practice of visiting graves to offer du'aa', perform salaah and read Qur'aan there. This is all bid'ah and is not prescribed in Islam.
(for details, check: http://islamqa.info/en/6744)