Answer
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.
In this fatwa:
Muslims are not allowed to turn to anyone, including the messenger of Allah, for help or imploring assistance or protection. If Muslims say Ya Muhammad (O Muhammad), that is shirk and that is contrary to the teachings of the Prophet.
Responding to your question, Sheikh Ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
If you were to ask for help, ask from Allah alone
Tawhid, or pure Oneness of Allah, is the core belief of Islam. That is why (in Surat al-Fatihah) we pray a minimum of 17 times a day to Allah, “You alone, we worship, and from You alone, we seek assistance.”
The Prophet (peace be upon him) taught this belief in his companions. Once he told Ibn Abbas, “Be Mindful of Allah, Allah will mind you….if you were to ask for help, ask from Allah alone; if you seek assistance, seek assistance from Allah alone.” (At-Tirmidhi)
Is it permissible for Muslims to say ‘Ya Muhammad’?
In the light of these and other clear evidence from the Quran and the Sunnah, we can readily conclude that we are not allowed to turn to anyone, including the messenger of Allah, for help or imploring assistance or protection. If we do so, that is shirk. That is contrary to the teachings of the Prophet (peace be upon him).
We may do well to remember that shirk is an unforgivable sin in Islam. So we must never harbor or condone such beliefs or practices.
Almighty Allah knows best.