Is The Introduction of Sahih Muslim Part of it?

Firstly:

What is known as the "Introduction to Sahih Muslim" refers to the pages that Imam Muslim wrote at the beginning of his book "As-Sahih" before he started narrating the Hadiths of the book, which (i.e., the introductory pages) amount to about 30 pages in the printed versions.

He did not give it a specific name, but began the content of the introduction without a title, where he said (may Allah have mercy on him):

"In the name of Allah, the Most Gracious, the Most Merciful, all praise is due to Allah, Lord of the worlds, And the [best] outcome is for the righteous, and may Allah send blessings upon Muhammad, the seal of the prophets, and upon all the prophets and messengers.

To proceed: You, may Allah bestow mercy on you by granting you success from your Creator, mentioned that you intended to examine the collection of reports transmitted from the Messenger of Allah(peace and blessings be upon him) concerning the Sunnahs of the religion and its rulings, and what there is of reward and punishment, encouragement and deterrence, and other types of matters with the chains of narration by which they were transmitted, and which the scholars have exchanged among themselves..." End quote from "Introduction to Sahih Muslim" (p. 2).

However, scholars have conventionally referred to these pages as "The Introduction," and some call it "Khutbat Al-Kitab."

Secondly:

The subject of this introduction addresses the following points:

First: Clarifying the reason for the book's composition.

Second: Explaining the subject of the book.

Third: Describing his methodology in the book.

Fourth: Discussing some issues of Hadith sciences that the reader of the book should be aware of.

Thirdly:

Is the introduction by Imam Muslim considered part of his book "As-Sahih"?

In terms of its narration and writing, it is not separate from "As-Sahih" because it has been narrated, copied, and printed as part of his book, and it has not been transmitted separately from it, so it is like any introduction or pologue of a book.

But in terms of its subject matter and content, it is not part of his book "As-Sahih"; rather, it is merely its introduction.

This is understood from the action of Imam Muslim, where he said after concluding the introduction:

"With the help of Allah, we begin, and we seek sufficiency in Him, and our success is only through Allah, Glorious is His Majesty." End quote from "Sahih Muslim" (1/28).

Then he began to mention the Hadiths of his book.

Scholars have conventionally distinguished between "The Introduction" and "As-Sahih." When they attribute a Hadith mentioned by Imam Muslim in "The Introduction," they do not usually say it was narrated by Muslim in his "Sahih," but rather they say it was narrated in "The Introduction to As-Sahih," as is evident from the practice of Al-Mizzi in his book "Tuhfat Al-Ashraf," and other scholars.

This distinction is clearly evident from what Al-Mizzi said in the introduction to his book "Tuhfat Al-Ashraf bi M`arifat Al-Atraf":

"... To proceed: I have resolved, Allah willing, to compile in this book the common portions of the six books which are the mainstay of the Muslims and upon which most rulings revolve, namely:

Sahih Muhammad ibn Ismail Al-Bukhari, Sahih Muslim ibn Al-Hajjaj An-Naysaburi, Sunan Abu Dawud As-Sijistani, Jami` Abu `Isa At-Tirmidhi, Sunan Abu `Abd Ar-Rahman An-Nasa’i, and Sunan Abu `Abdullah Ibn Majah Al-Qazwini, and what is similar to them, such as: The Introduction to the book of Muslim..." End quote from "Tuhfat Al-Ashraf" (1/3 - 4).

Likewise, they differentiate between the narrators mentioned in the introduction and those whose Hadiths Imam Muslim narrated in the main body of his book "As-Sahih."

The narrator whose Hadiths Imam Muslim only narrates in the introduction is described as being among the men of Muslim in "The Introduction," as Al-Mizzi did in his book "Tahdhib Al-Kamal," where he said in the introduction to his book, defining the symbols of his book:

"The symbol for what Muslim narrated in As-Sahih is: (M), and the symbol for what he narrated in the introduction to his book is: (MQ)" End quote from "Tahdhib Al-Kamal" (1/149).

Those who came after him did something similar in the biographies of the narrators.

Ibn Al-Qayyim (may Allah have mercy on him) said:

"... As for your saying: Muslim narrated from Sufyan ibn Husayn in his "Sahih," it is not as you mentioned, but rather he narrated from him in the introduction to his book. Muslim did not stipulate in it what he stipulated in the book regarding authenticity, so it has its own status, and the rest of his book has another status, and the people of Hadith have no doubt about this." End quote from "Al-Furusiyya Al-Muhammadiyya" (1/183).

And Allah knows best.