What is the ruling on directing supplication to the Prophet (peace and blessings be upon him)?

Question:
This person is asking about the one who supplicates to Muhammad, calling the Messenger, saying "O Muhammad, O Messenger of Allah," this is...

Answer:
It is not permissible. The Messenger is not to be supplicated to; rather, Allah (Exalted and Majestic is He) is the one to be supplicated to. The Messenger is not to be supplicated to because he has no share in lordship and no share in worship. Worship is the right of Allah, and supplication is the right of Allah. So the right of the Messenger upon you is to follow him, and the right of Allah upon you is to worship Him.
He alone has no partner. You must distinguish between the right of Allah and the right of the Messenger (peace and blessings be upon him). The right of the Messenger is following him, loving him, and obeying him. The right of Allah (Mighty and Majestic) is worship. It is not permissible to associate with Allah in His worship anyone, whether from the prophets, the angels, the awliya (close friends of Allah), or the righteous. Do not associate [with Him] and do not associate anything with Him. It is not permissible to combine Allah and the Messenger in supplication or in worship; this is shirk (associating partners with Allah), and I seek refuge in Allah from that.