Praise be to Allah, and may peace and blessings be upon the Messenger of Allah. To proceed:
First: Asking the Prophet ﷺ to make du‘a after his death is not prescribed
For a person to say at the grave of the Prophet ﷺ:
“O Messenger of Allah, ask Allah to forgive me,”
or:
“Pray to Allah for me,”
is something that the Companions (may Allah be pleased with them) did not do after the death of the Prophet ﷺ, even though they loved and honored him more than anyone else.
Therefore, when the Muslims suffered from drought during the time of ‘Umar (may Allah be pleased with him), they did not go to the grave of the Prophet ﷺ to ask him to make du‘a for them. Instead, ‘Umar asked Al-‘Abbas (may Allah be pleased with him), who was alive, to supplicate to Allah for them.
This indicates that asking the Prophet ﷺ to make du‘a after his death is not part of the Sunnah.
Second: Is this act shirk?
This matter requires some clarification.
If what is meant is:
That the Prophet ﷺ hears the supplication and possesses, by himself, the power to fulfill needs, relieve distress, or control the affairs of the universe, then this constitutes major shirk.
However, if what is meant is merely asking him ﷺ to make du‘a or intercede, without believing that he possesses any of the exclusive attributes of Lordship, then according to many scholars, this is:
A prohibited religious innovation (bid‘ah).
Not major shirk.
For this reason, a number of scholars have stated that saying:
“O Messenger of Allah, pray to Allah for me,”
after his death ﷺ is an unprescribed innovation, but it does not amount to major shirk as long as the person does not attribute to him any qualities of divinity or Lordship.
Third: Why is this prohibited?
Because worship, supplication, and the heart’s ultimate dependence and attachment must be directed to Allah alone.
After the death of the Prophet ﷺ, he is not to be asked to make du‘a, and this practice was not prescribed for the Muslim community.
All goodness lies in following the way of the Companions (may Allah be pleased with them).
Summary
Asking the Prophet ﷺ to make du‘a after his death is not prescribed.
The Companions (may Allah be pleased with them) did not do this.
If the request is merely asking him to make du‘a without believing that he possesses independent power or control, then according to many scholars it is a prohibited innovation and not major shirk.
However, if a person believes that the Prophet ﷺ independently possesses the power to bring benefit, cause harm, or fulfill people’s needs, then this constitutes major shirk.
What is prescribed when visiting his grave ﷺ is to greet him with salam, invoke Allah’s blessings upon him, and direct supplication to Allah alone.
And Allah knows best.