There are many ahadeeth which have this denotation I will quote two of them
عن أبي موسى عن النبي صلى الله عليه وسلم قال من حمل علينا السلاح فليس منا
1) The one who takes up arms against us, is "Not from us" [Saheeh Bukahri: 7071]
2) The one who cheats is "Not from me"(من غش فليس مني ) [Saheeh Muslim: 102]
Now when laymen read this hadeeth or any other which has this term might interpret that the one who does these acts does not remain Muslim. Because the apparent meaning indicate towards it, while in reality its not like that rather this was the interpretation of Khawarij (deviant group).
Shaykh ul Islaam Ibn Taymiyyah rahimahullah said
وكذلك تفسير الخوارج والمعتزلة بأنه يخرج من الإيمان بالكلية ويستحق الخلود في النار تأويل منكر
And likewise the interpretation of Khawarij and Mu'tazila (with the denotation Laisa Minna that the one who does it) is out of faith completely and deserves to be in hell forever then this is a rejected interpretation [Majm'oo al Fatwaa 7/525]
Now the question If he remains Muslim then what does "Not From us" mean? and what is the ruling on act mentioned with it?
Meaning of Denotation (Laisa Minna)
When this denotation is used then it would mean "Not Completely over our Sunnah or our methodology or our way"
Ibn Hajr asqalani rahimahullah comments on the chapter of Saheeh Bukhari (باب ليس منا من شق الجيوب) Chapter: The one who tears his clothes is "Not from us" and says
قوله ليس منا أي من أهل سنتنا وطريقتنا وليس المراد به إخراجه عن الدين ولكن فائدة إيراده بهذا اللفظ المبالغة في الردع عن الوقوع في مثل ذلك كما يقول الرجل لولده عند معاتبته لست منك ولست مني أي ما أنت على طريقتي
Saying of Prophet (صلى الله عليه وسلم ) "Not From us" (then) it means "Not from our Sunnah or our way" it does not mean (that the one who does it) is expelled from religion and on the contrary the benefit of him referring this word (i.e denotation) is to intensify condemnation over occurence of such things, like (when) a person says to his son while rebuking I am not from you and you are not from me, (then) it means you are not on my way
.[Fath al Baari 3/163]
Ibn al Jawzi rahimahullah while commenting on the hadeeth of Saheeh Muslim (He who learnt archery and then neglected it, is not from us, (or said) He has been guilty of disobedience) says:
قوله ليس منا أي ليس على سيرتنا وهذا لأن الرمي من آلة الجهاد فإذا تركه من علمه نسيه
His saying "Not from us" (it means) "Not on our way and this is because archery is tool of jihaad so, when the one who knows it leaves (i.e stop practicing) it forgets it. [Kashf al Mushkil min Hadeeth as Saheehayn 1/1111]
Imam Nawawi rahimahullah says regarding this denotation
ومعناه عند أهل العلم أنه ليس ممن اهتدى بهدينا واقتدى بعلمنا وعملنا وحسن طريقتنا كما يقول الرجل لولده إذا لم يرض فعله لست مني وهكذا القول في كل الأحاديث الواردة بنحو هذا القول
Meaning of denotation (laisa minna) near people of knowledge is (that the one who does that act) is not from those who are guided to our way or not from the followers of our knowledge and actions or not from those who follow the splendidness of our way, like a person to says to his son when he does not accede his action, you are not from me. This is the saying for all the ahadeeth which has come with this denoatation [ Sharah Saheeh Muslim 1/109]
Ruling on the acts mentioned with this denotation.
The denotation "Not From us" is apparently (البراءة) disavowal and absolute disavowal is only from Kufr(that's the reason khawarij deduced with it over their Takfeer) but a conditional disavowal from the one who believes in Allah and his Messenger apparently indicate towards prohibition(of the act mentioned) and a conditional disavowal over a thing which is not prohibited evince towards Makrooh Tanzeehi (dislike) or leaving a preferable act.
To rule out all the acts mentioned with this denotation to be prohibited is not correct either because it is has been used for other than prohibition too. I will quote two hadeeth
(ليس منا من لم يتغن
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