Answer
Asalamu Alaikum Aisha,
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Muslims believe that Prophet Muhammad (PBUH) was the last prophet and messenger of God. In Islam a prophet or a messenger conveys the will of God for a nation or all humankind.
The message delivered by the prophet is binding on those to whom it is sent. And a rejection of the prophet is a rejection of God. A messenger may bring a change to earlier religious laws and create a new religious community.
The belief that Prophet Muhammad (PBUH) is the last prophet and messenger of God means after him there will not arise any person who will be authorized by God to express God’s will for others in a binding way and/or institute a new religious direction by a new expression of the religious truth and forming a religious community around that expression.
Any person claiming to have such authority is suffering from self-deception and/or is lying; no matter how smart he may be or how many miraculous deeds he may perform.
Proof from the Quran
The Quran says:
{Muhammad is not the father of any male among you, but he is the messenger of God and the seal of the prophets; and God is aware of all things ,} (Quran 33:40)
The Arabic word for seal is “khatam” which, by a change of vowel, can also be read as “khatim”, meaning “that which puts the seal”.
Both words are derivatives of “khatama”, which means both to end or conclude something or to put a seal in order to indicate such an end or conclusion (see for example, Lisan al-‘Arab).
Muslims understand the verse to mean Muhammad (PBUH) completed and closed the prophethood, as a seal marks the completion and closure of a document. That is to say, he was the last prophet. This interpretation is also supported by the reference to the Prophet not leaving behind any son.
Arab society prior to Islam considered it important for a man to have a son. The Arabs believed a man only attained posterity through a son. A person without a son was called “abtar” (one who is cut off).
The Makkans applied this description to the Prophet (PBUH) because none of his sons lived to puberty. Regarding this the Quran says:
{Verily, We have granted you Al-Kauthar; Therefore, turn in prayer to your Lord and sacrifice; for he who hates you, he will be cut off (abtar).} (Quran 108:1-3)
Now the meaning of the title or description “seal of the prophets” becomes clear with the reference to the absence of a male descendant of the Prophet.
Muhammad (PBUH) may not live on through his male descendants (posterity), but he will live on forever in a much more important way. Because of its finality, his prophethood will last forever, and will be a source of everlasting and abundant blessings.
In support of this understanding, we find the Quran never looks forward to a future revelation or prophet. For example:
{(The pious are) those who believe in what is sent down to you (O My Prophet) and in what was sent down before you.} (Quran 2:4)
There is no reference to what will be sent down after the Prophet. Nowhere else does the Quran refer to a future prophet or future revelation.
One of the last verses of the Quran to be revealed states:
{[…] Today, I have completed My religion for you and perfected My favor on you and chosen al-Islam as your way of life.} (Quran 5:3)
Here, we are told that the revelation from God is complete and perfect. That means there are no omission and no commission; therefore, no need for any more revelation.
In fact, the Quran regards itself as fulfilling an earlier prophecy:
{Say, whether or not you believe in it, the fact is that those who possessed knowledge before it fall on their faces in humble prostration when it is recited to them. And they say, Glory to our Lord and Sustainer: Surely the promise of Our Lord and Sustainer was to be fulfilled!} (Quran 17:107-108; see also, 7:157, 61:6)
Nor does it prophesy a later revelation:
{He it is who has sent His messenger with the guidance and the religion of truth that He may make it prevail over all religion, however much those who practice polytheism may be averse.} (Quran 61:9)
Islam is the clearest and most effective expression of that truth, and; therefore, is destined to replace all other expressions.
Some people have tried to interpret the words “seal (or last) of the prophets” to suit their own agenda, but none of these interpretations are supported either by the Quran or hadith. As a result, we have seen false prophets.
Any interpretation of the expression must explain why it is combined in the Quran with the observation that the Prophet had no male descendant.
Some people may pick on the word prophet in “seal (or last) of the prophets” and say, but what about a messenger? There is a difference between a messenger and a prophet. A simple explanation is: a messenger comes with a message from God, while a prophet comes to confirm the existing message from God. All messengers are prophets, but not all prophets are messengers.
We can conclude that if there is no prophet after Prophet Muhammad (peace be upon him), then there is also no messenger after him.
Walaikum Asalam.
Please continue feeding your curiosity, and find more info in the following links:
Why Did Muhammad Come After Jesus?
The Timelessness of Prophet Muhammad’s Message
6 Evidences of Muhammad’s Prophethood
What Is the Proof that Muhammad Was a Prophet?