Excerpt from the chapter on the Mahdi Prophet from the book The Awaited Messages

(The Mahdi will be sent by Allah (swt) to the Ummah)

Part of the answer to the recurring question to me is why didn't the Prophet tell us that a new messenger would be sent?
I will now publish part of the answer to this question. The full answer contains several points, including that the Prophet (peace be upon him) preached the Mahdi to us in several hadiths, just as the Prophet (peace be upon him) preached Jesus to Muhammad (peace be upon him), and the Prophet (peace be upon him) described the Mahdi to us, which did not happen with Saladin and Qutuz, for example, and told us about his deeds and miracles that will happen during his reign.
But here I will quote the part about the Prophet telling us that the Mahdi will be sent by Allah (swt) to us, so here is part of the answer, and those who want more evidence should read the book because I will not be able to transfer the book or summarise it here.

(The Mahdi will be sent by Allah (swt) to the Ummah)

On the authority of Abdul Rahman ibn Auf from his father, he said: The Messenger of Allah, may Allah bless him and grant him peace, said: "Allah will send a man from my family who will fill the earth with justice, who will fill the earth with justice, and will overflow the money."
On the authority of Abu Sa'id al-Khudri, the Messenger of Allah, may Allah bless him and grant him peace, said: "The Mahdi will emerge in my nation, whom Allah will send as a relief for the people, and the nation will prosper, the livestock will live, the earth will bring forth its plants, and the money will be given freely."
Abu Saeed al-Khudari said: The Messenger of Allah, may Allah bless him and grant him peace, said: "I announce to you the Mahdi, who will be sent in my ummah at a time when people will be divided and there will be earthquakes, and he will fill the earth with justice and righteousness as it was filled with injustice and oppression, and the inhabitants of heaven and earth will be pleased with him. A man said to him: "What is right?" He said: "The righteousness between the people.
The word "resurrection" here has very important connotations, the most important of which is sending. In most of the hadiths narrated by the Prophet (peace be upon him), the word "resurrection" is used in the sense of sending. For example, Sahl bin Saad (may Allah be pleased with him) reported that the Messenger of Allah (peace be upon him) said, "I and the hour were sent like this": "I was sent like this," he said, pointing with his two fingers and extending them, and the Messenger of Allah said, "I was sent to perfect good manners" [narrated by Ahmad]; and it is proven from the Prophet (peace be upon him) that he said, "The best of the centuries is the century in which I was sent, then those who follow them, then those who follow them." This is proven in the two Sahihis from different sides.
The same term was used by the Prophet (peace be upon him) in relation to the return of Jesus at the end of time, in Sahih Muslim after mentioning the Dajjal sedition: "While he was like that, Allah sent the Messiah, the son of Mary, and he would descend at the White Minaret east of Damascus between two mahrudas, with his palms resting on the wings of two angels..."
Therefore, the term is clear and used in the time of the Prophet (peace and blessings of Allah be upon him) a lot, and most of its usage is in the sense of sending, i.e. being sent by Allah (swt) or being sent by someone else, so that the one who is sent is called a messenger. If the Prophet (peace and blessings of Allah be upon him) knew that this well-known term in the sense of sending would cause strife for the Muslims in the future. If the Prophet (peace be upon him) had known that this popular term in the sense of sending would cause strife for Muslims later on, he would not have used it with the mention of the Mahdi and Jesus (peace be upon him) attached to the name of Allah (swt), and would not have left us confused about the meaning of the resurrection, the Prophet (peace be upon him) could have said "a man will appear or come from my family" and not "Allah will send a man from my family...The word "resurrection" is frequently used in the hadiths of the Mahdi, there is a verbal frequency that Allah will send the Mahdi in more than one prophetic hadith, and the same is the case with Jesus, peace be upon him, "...when Allah sent the Messiah, son of Mary...".
In order to understand the meaning of the Prophet's hadith about God's resurrection of the Mahdi, we must understand the meaning of resurrection in the language, from which you can judge what is meant by the phrase "God will resurrect the Mahdi" or "God will resurrect Jesus." In the book The Decalogue Encyclopedia, the concept of resurrection is as follows:

The definition of resurrection in language varies depending on what is attached to it:

1- Sending: It is said, "I sent a person or sent him, i.e. I sent him." Ammar bin Yasser (may Allah be pleased with him) said, "The Prophet (peace be upon him) sent me on a need and I impregnated myself and could not find water, so I wallowed in the grass as the animal wallows." ..... [Agreed upon].
2- Resurrection from sleep: It is said: He resurrected him from his sleep if he woke him up (this meaning is not consistent with the Mahdi's status and mission).
3- Excitation: This is the origin of emission, from which it is said of a camel: Al-Azhari says in Tahdhib al-Language: "Al-Layth said: "I sent a camel to be resurrected if I unhooked it and sent it out, if it had been blessed, I would have resurrected it.
He also said: The resurrection in the Arabic language has two faces: One of them: Sending, as Allah says: "Then We sent after them Moses and Aaron to Pharaoh and his people with Our signs, but they were proud and were criminal people" [Yunus], meaning that We sent them.
Resurrection is also the resurrection of the dead by God, hence the saying: "Then We resurrected you from your death so that you may be thankful" [Al-Baqarah: 56], i.e. we revived you.
Abu Hilal said: The resurrection of creation: A name for bringing them out of their graves to the parking lot, from which comes the saying: 'They said, 'O woe is us, who has resurrected us from our graves.

The quotation is from the book The Awaited Messages, the chapter of the Mahdi Prophet Zalman who wants more evidence should read the book.