Bukhari

Hadith Commentary:
Imam Bukhari has derived the word "muqtasimeen" from "qasm" (oath).
Some have said that it is derived from "qismat" (division), which means "those who divide," that is, those people who tore the Qur'an into pieces, who broke it into fragments.
Several meanings have been stated for this: one is that some called the Prophet a sorcerer, some called him mad, some called him a soothsayer.
Secondly, that they mocked the Qur'an.
Mujahid said that the Jews are meant, who believed in some parts of Allah's Book and did not accept some parts.

Hadith Commentary:
1.
In these hadiths, by ﴿مُقْتَسِمِينَ﴾ is meant the Jews and Christians. Indeed, they created dozens of sects by accepting some parts of their religious book and rejecting others, as well as by concealing certain verses and altering others.
2.
Some scholars say that by this is meant the disbelievers of Makkah, whose demand regarding the Noble Qur’an was that the verses in which their idols are insulted should be removed, and the rest of the verses they would accept. Allah Ta’ala sent His scourge of punishment upon all of them—whether they were the People of the Book or the people of Makkah—Allah Ta’ala did not forgive those who possessed such conduct.