What is the authenticity of the Hadith: "Whoever walks with an oppressor to assist him, knowing that he is an oppressor, has left Islam," and is assisting an oppressor considered one of the nullifiers of Islam, and have any of the Imams stated this?

The hadith: "Whoever walks with a ẓālim (oppressor) to assist him, knowing that he is an oppressor, has departed from Islam" is very weak and not authentic. Even if it were authentic, its interpretation would be that assisting in oppression removes one from the perfection or reality of faith (īmān), but not from Islam itself. Doing so does not expel one from the millah (religious community). Oppression is of different types: some types expel one from the millah, such as shirk (polytheism), while others do not, such as oppressing others through sins and transgressions. There is no one among Ahl al-Sunnah wal-Jama'ah who declares people disbelievers due to sins; rather, they have agreed unanimously that a Muslim is not declared a disbeliever for them, except for what the Shar' (Islamic law) explicitly states as disbelief. Assisting an oppressor is forbidden and is considered participation with him in his sin, making one liable for severe punishment.