Answer
Wa `alaykum as-salamu wa rahmatullahi wa barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Dear brother, we thank you for your question and we commend your eagerness to become well-acquainted with Islam and its teachings, which is the way Allah has chosen for the welfare of His people.
The term saint is sometimes used as a translation for the Arabic word “wali” which refers to a good, righteous person who is faithful and devoted to Allah. Throughout Islamic history, such true Muslims set great examples of sincerity and loyalty to their faith; their dedication and commitment are worth of imitation by all Muslims.
However, that does not grant them any type of divinity or make them worthy of any sort of worship. They are human beings, just like anybody else.
Therefore, it is not acceptable to build tombs on the graves of the pious or direct any kind of worship or supplication to them. Only Allah the Almighty, the lord and the Creator, is the One Who deserves worship.
In his response to your question, Sheikh Ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states,
Awliya’ is the plural form of wali; which means friend, protector, guardian, etc. In Islamic literature, it refers to those who have been considered as truly pious and God-fearing and thus serve as role models for others. We read in the Qur’an, (Verily, the friends “Awliya’” of Allah have nothing to fear or grieve for.) Then the next verse goes on to define the friends of Allah: (They are they who cherish true faith (in Allah) and thus remain conscious (of Him). And such are the ones who shall receive glad tidings from Allah in this world and the next…) (Yunus 10:62-64)
So, the friends of Allah are those who are pious and God-fearing. The foremost of this group are the messengers of Allah, who serve as the role models for humanity. They are followed by those who imitate them.
In Islam, next to the Prophet Muhammad (peace and blessings of Allah be upon him) in saintliness come the four righteous caliphs and the eminent Companions as well as the pious forbears and imams. There was no dearth of awliya’ Allah in the history of Islam; thanks to their efforts, Islam has reached all corners of the globe.
As for the practice of building tombs on the graves, it is contrary to the explicit teachings of Islam; for the Prophet (peace and blessings of Allah be upon him) has sounded grave warnings against turning graves into places of excessive veneration.
Ironically, the Muslims have built tombs on graves of saints and turned them into places of excessive veneration. This is undoubtedly a reprehensible innovation to be rejected.
Allah Almighty knows best.