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 questioner, we would like to thank you for the great confidence you place in us, and we implore Allah Almighty to help us serve His cause and render our work for His Sake. May Allah reward you abundantly for your interest in knowing the teachings of Islam!
First of all, we’d like to state that a Muslim believes that whatever has been written by the pen in the heavens must pass. What will happen cannot be changed. If our beloved Prophet had to pass away, then, of course, even if all the heavens and earth joined together with the all the people and every kind of living thing and prayed to Allah to heal the Prophet, their prayer would not be accepted because whatever has been written has to pass.
In fact, every committed Muslim feels indebted to Prophet Muhammad (peace and blessings be upon him). It is through him that Allah’s light and guidance reached us. He is the last Prophet and Messenger who was sent by Allah to the world of humanity. He preached Monotheism and continued the authentic message of the truthful prophets like Adam, Noah, Abraham, David, Solomon, Moses, and Jesus. The Noble Prophet passed away, as all prophets did.
With regard to your question, it is to be noted that the Messenger (peace and blessings be upon him) was a human being who suffered from sickness and real death the same as any other human being.
Answering your question, the prominent Muslim scholar, Sheikh `Abdul-Kareem Al-Khudayr, states:
The Messenger (peace and blessings be upon him) is the best and most perfect of creation, and the most beloved to Allah and the most honored by Him. But this does not mean that his human characteristics are to be denied, or that any of the duties of worship which are due only to Allah should be directed to him. The Messenger (peace and blessings be upon him) was a human being who suffered from sickness and real death the same as any other human being.
Almighty Allah says, “Verily, you (O Muhammad) will die, and verily, they (too) will die” (Az-Zumar 39:30)
He says also, “And We granted not to any human being immortality before you (O Muhammad); then if you die, would they live forever?” (Al-Anbiya’ 21:34)
The Prophet (peace and blessings be upon him) died and was buried in his grave, hence as-Siddiq Abu Bakr (may Allah be pleased with him) said, “Whoever used to worship Muhammad, Muhammad has died, but whoever used to worship Allah, Allah is alive and will never die.”
The fact that the Prophet (peace and blessings be upon him) was a witness, a bringer of glad tidings and a warner, and that he will be a witness on the Day of Resurrection, does not mean that he is present among all nations, or that he (peace and blessings be upon him) will remain alive until the Day of Resurrection, or that he sees and witnesses whilst he is in his grave. Witnessing does not only mean seeing. Rather he will testify against the nations by means of what Allah tells him, for he does not know the Unseen.
Allah, Most High, says, “… and if I had the knowledge of the unseen, I should have secured for myself an abundance of wealth …” (Al-A`raf 6:188)
The Prophet (peace and blessings be upon him) does not have the ability to be present in numerous places; he is in only one place, which is his grave, and this is according to the consensus of the Muslims.
Excerpted, with slight modifications, from: www.islamqa.info
We end our discussion by quoting the Qur’anic verse that reads, “Muhammad is no more than a Messenger, indeed (many) Messengers have passed away before him. If he died or is killed, will you then turn back on your heels (as disbelievers)? …” (Al `Imran 3:144)
Allah Almighty knows best.