Sheikh `Abd al-Rahmân al-Barrâk
It is polytheism to make supplications to the Prophet (peace be upon him) or a pious person and ask him for things that are needed. This includes requesting them to offer supplications on your behalf. This is unlawful and it is pure polytheism.
Seeking the supplications of living pious people on your behalf is allowed, such as to approach a pious man – during his life time but not after his death – and ask him to pray for you. The Companions used to ask the Prophet (peace be upon him) to make supplications for them.
However, it is not permissible to seek supplications from those who have died or from those who are not present. The Companions stopped asking the Prophet (peace be upon him) to make supplications on their behalf after the Prophet (peace be upon him) had died. In the year of famine, `Umar said the following: “O, Lord, we were soliciting you by our Prophet and you would answer us and let us have rain, now we are soliciting you by the uncle of our Prophet, so we ask you to answer us.” Then he asked the Prophet’s uncle to supplicate on their behalf.
The Companions clearly made a distinction between the living and the dead in this matter.
Asking for the Prophet's intercession on the Day of Judgment directly from the Prophet (peace be upon him) in our supplications is an innovation.
The Companions used to ask the Prophet (peace be upon him) to make supplications for them when he was alive. Even then, their asking him for his intercession was never once cited.
This is actually the Prophet's status on the Day of Judgment which is solely for him. A person may not use the intercession of the Prophet (peace be upon him) or other pious people because the intercession of person is an honor bestowed upon him by Allah on the Day of Judgment.
Asking for intercession is not akin to polytheism, but is an innovation in religion. What is polytheism is to make supplications to the Prophet (peace be upon him) or a pious person and ask them for things that are needed or for their supplications. This is, as we said before, pure polytheism.
Source: Islam Today