Assalamu Aleykum,
The famous Shia Ayatollah Muhammad Hassan Al- Najafi (died in 1266 A.H) in his volumnous book Jawaher Al- Kalam (a reference in the Fiqh) Vol 22 p. 62 said:
" It is apparent that those who differ with us are associated with non-believers (Mushrikeen) in that matter (i.e Gheebah)because the Kufr of Iman and Islam are combined in him , rather maligning them infront of the people is one of the best acts of worship , unless Taqqiyyah would prevent it. Even better than that is their Gheebah (to speak ill of someone behind his back), an act that the Shia' performed at all times and in all places, both their scholars and laymen, such that they have filled sheets from it and it is considered by them as one of the best acts of worship and most complete of acts getting one closer to Allah. Hence, it is not strange to find claims of consensus on this matter from some (scholars) , even more it could be considered one of the essentials (of religion) less to say qateyyat (matters that are affirmed).
And what has been mentioned of Al-Muqqada Al-Ardabilil and Thahir Al-Khurasani that the general proofs that prohibit (Gheebah) from the Book and Sunnah for the believers and others is strange. (They say) "that the saying of : " And do not speak ill of (wa la yaghtab)" to the end of the (ayah) is with respect to all sane people (Mukalafeen) or for Muslims, because it is permissible to make gheebah of Kafir and most of the Sunnah (i.e the traditions attributed to the Imams of ) has the expressions of " people" and "Muslim" to cover everybody, and this is not strange for as it is not permissible to take the money of those who differ with us or kill them, so is speaking ill if him."
And then he (i.e either Muqaddas or Thahir) said: " I think that Al-Shaheed in his fundamentals has made permissible the Gheebah of those who differ with us concerning their Madhab and religion and none else"
And as you see this saying is contrary to what you have heard (earlier - i.e to what Al-Najafi has reported at the beggining of this quote) , and it may be that this has occured from him due to him trying to be very holy and God-fearing, but it cannot be hidden from the expert who has come across many proofs that reach tawatur of cursing them (i.e Sunnis), abusing them, insulting them, (stating) their Kufr, and that they are the Majus of this Ummah, that they are worse than the Christians , and more filthy than the dogs and trying to be holy and God-fearing dictates otherwise.
Furthermore, the beggining of the ayah is addressing "those who believe" and it ends with an analogy with eating the flesh of one's brother. Even in Jami Al-Maqasid the definition of Gheebah as reported in traditions is to say true things about one's brother which he hates if he heard it. And it is known that has established brotherhood between the believers by His -tala- saying: " Believers are but brothers" and this does not include others , for how can brotherhood be considered between a believer (i.e Shiite) and one who differs with us , even after the tawatur (i.e multitude) of traditions and verses in the obligation of showing enmity towards them and having "Tabarrah" from them and hence the expressions of "people" and "Muslim" should (only) be associated with the believers from amongst them as reported in four traditions.
And how big a contrast is between what he said and what Khaja Nusair-el-deen Al-Tusi and Alam Heli and others (believe) in killing them and such charachterisitcs of Kafir as happened in Baghdad and its surroundings.
And to conclude, giving lengthy discussions about this topic , as he did in Al-Hadaeq, is a waste of lifetime in clear matters, for the least that could be said is the permissibility of their Gheebah because of their open Fisq for what they are doing and believe in is one of the greatest kinds of Fisq, rather Kufr, and if they have been treated as Muslims in some verdicts out of necessity, and you will (shortly i.e in the coming chapters of the book) know that one of does Fisq openly is not immune from Gheebah for the thing he (declared) and for other things. Accordingly, one knows the fallacity of what Al-Shaheed mentioned and in any case it has become obviuos that the immunity (hurmah) is only given to the believers who believe in the 12 Imams and none other, be it a Kafir or one who differs with us even if he does not acknowledge one of the (Imams) -AS-."
source : brother al-A3sha from islamic-forum.net
وعلى كل حال فالظاهر إلحاق المخالفين بالمشركين في ذلك لاتحاد الكفر الاسلامي والايماني فيه، بل لعل هجاؤهم على رؤس الاشهاد من أفضل عبادة العباد ما لم تمنع التقية، وأولى من ذلك غيبتهم التي جرت سيرة الشيعة عليها في جميع الأعصار والأمصار علمائهم واعوامهم، حتى ملأوا القراطيس منها بل هي عندهم من أفضل الطاعات، وأكمل القربات فلا غرابة في دعوى تحصيل الاجماع، كما عن بعضهم بل يمكن دعوى كون ذلك من الضروريات، فضلا عن القطعيات، فمن الغريب ما عن المقدس الأردبيلي وظاهر الخراساني في الكفاية من أن الظاهر عموم أدلة تحريم الغيبة من الكتاب والسنة للمؤمنين وغيرهم لأن قوله تعالى (1) (ولا يغتب) إلى آخره للمكلفين أو للمسلمين، لجواز غيبة الكافر والسنة أكثرها بلفظ الناس والمسلم وهما معا شاملان للجميع، ولا استبعاد في ذلك إذ كما لا يجوز اخذ مال المخالف وقتله، لا يجوز تناول عرضه، ثم قال في ظني ان الشهيد في قواعده جوز غيبة المخالف من جهة مذهبه ودينه، لا غير إذ هو كما ترى مخالف لما سمعت، ولعل صدور ذلك منه لشدة تقدسه وورعه، لكن لا يخفى على الخبير الماهر الواقف على ما تظافرت به النصوص، بل تواترت من لعنهم وسبهم وشتمهم وكفرهم وانهم مجوس هذه الامة، واشر من النصارى وانجس من الكلاب، أن مقتضى التقدس والورع خلاف ذلك، وصدر الآية الذين آمنوا وآخرها التشبيه بأكل لحم الاخ بل في جامع المقاصد أن حد الغيبة على ما في الأخبار أن يقول في أخيه ما يكرهه لو سمعه مما فيه، ومعلوم أن الله تعالى عقد الاخوة بين المؤمنين بقوله تعالى (2) (انما المؤمنين اخوة) دون غيرهم، وكيف
(1) و (2) سورة الحجرات الآية 12 - 10
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يتصور الاخوة بين المؤمن والمخالف، بعد تواتر الروايات وتظافر الآيات، في وجوب معاداتهم، والبرائة منهم، وحينئذ فلفظ الناس والمسلم، يجب ارادة المؤمن منهما، كما عبر به في أربعة أخبار. وما أبعد ما بينه وبين الخاجا نصير الدين الطوسي والعلامة الحلي وغيرهم ممن يرى قتلهم، ونحوه من احوال الكفار، حتى وقع منهم ما وقع في بغداد ونواحيها، وبالجملة طول الكلام في ذلك كما فعله في الحدائق من تضييع العمر في الواضحات، إذ لا أقل من أن يكون جواز غيبتهم لتجاهرهم بالفسق، فان ما هم عليه أعظم انواع الفسق بل الكفر، وإن عوملوا معاملة المسلمين في بعض الأحكام للضرورة، وستعرف انشاء الله أن المتجاهر بالفسق لا غيبة له فيما تجاهر فيه وفي غيره، ومنه يعلم فساد ما حكاه عن الشهيد، وعلى كل حال فقد ظهر اختصاص الحرمة بالمؤمنين، القائلين بامامة الائمة الاثنى عشر دون غيرهم من الكافرين والمخالفين ولو بانكار واحد منهم عليهم السلام.
source :
http://www.yasoob.com/books/htm1/m001/03/no0318.html