Imaam Abu Muhammad Al-Hasan b. `Alee b. Khalaf al-Barbahaaree (died 329H, rahimahullaah) stated in his Sharhus-Sunnah (point 152):
If you see a man mentioning with good Ibn Abee Du’aad, Bishr al-Mireesee, Thumaamah, Abu Hudhayl, Hishaam al-Footee, or [praising] any of their followers and adherents, then be aware of him, for surely he is a person of innovation. Indeed these people were upon heretical apostasy – so abandon this man who speaks good of them, and the ones he is speaking good of.
وإذا رأيت الرجل يذكر ابن أبي داؤد والمريسي أو ثمامة وأبا الهذيل وهشام الفوطي أو واحدا من أتباعهم وأشياعهم فاحذره فإنه صاحب بدعة وإن هؤلاء كانوا على الردة واترك هذا الرجل الذي ذكرهم بخير ومن ذكر منهم
Shaikh Saalih al-Fawzaan commenting upon this stated in It-haaf al-Qaaree (vol. 2, p. 236):
“If you see a man praising the people of evil and the scholars of misguidance, such as these offshoots of the Jahmiyyah, then know that he is a sinner (faasiq), a corrupt and misguided person. This is because he would not praise such people except due to his love of them and his approving and tolerating their path. And if you see a man praising Ahlus-Sunnah such as Imaam Ahmad (d. 241H), Ibnul-Mubaarak (d. 181H) – and likewise praising the Scholars of the Taabi`een and those who came after them; then know that he is person of goodness. This is because he would not not praise Ahlus-Sunnah except due to his love of the Sunnah and holding fast to it.
So this is a lesson for us, in that some brothers or students of knowledge praise some of the innovators or people of desires and those who have deviated ideologies. So this [student] fails to look towards their [deviated] ideologies and orientations, and thus falls in attacking the people of goodness, and belittling them. This is because he listens to them criticising the people of good an he believes them. This is a great danger when he belittles the people of goodness, and belittles the scholars and ahlus-Sunnah, whilst he praises the deviated ideologies and orientations – this is a great danger! And this is even if he does not sit with the innovators himself. So this is what al-Barbaharee warns us from – this affair into which many of the people of our times have fallen into.
The author (Al-Barbahaaree) said: “Ibn Abee Du’aad  and Bishr al-Mireesee ..” These are the two that encouraged the ruler, Al-Ma’moon, to inflict punishment upon Imaam Ahmad (rahimahullaah) and upon others from the Scholars so as to extract from them the saying that the Qur’aan is created. Thumaamah Ibnul-Ashras  was from the leaders of the people of misguidance.
Abu Hudhail al-`Allaaf  was from the major heads of the Mu`tazilah. Hishaam al-Footee  was also from the innovators.
Al-Barbahaaree (rahimahullaah) said, “or [praising] any of their followers and adherents, then be aware of him” Meaning: If you see him praising the people of evil, and deviation then be aware of him.
His saying: “For indeed they were upon apostasy..” Meaning: Some of them were outright apostates – they were the Imaams of the Jahmiyyah and the Mu`tazilah. Those who deliberately and knowingly opposed the Book and Sunnah. Concerning them, there is no doubt about their unbelief. As for one who blindly follows them, then he is judged to me misguided. Such a person is not judged as being an unbeliever until the matter is clearly explained to him. As for their Imaams, and their callers, then they know what they are upon of misguidance, and so for that reason they are judged with the ruling of apostasy.
His saying: “So abandon this man who speaks good of them.” Do not be deceived by this man who extolls their virtues and praises them. It is possible that some of the people of deviations have some good traits, however, look to what they possess of misguidance, and so do not be deceived by a trait from the traits of goodness, and thus you become inattentive concerning their many evil characteristics. And this also is a great wisdom because some people say: “So and so has goodness”. However, if he is a deviant, there is no good in him, just like a person of Sunnah – even if there is with him some small amount of evil, stick with him, because he is a person of Sunnah.
Footnotes: Imaam Adh-Dhahabee (rahimahullaah) said: “Ahmad Ibn Abee Du’aad al-Qaadee – a detestable, loathsome Jahmee. Destroyed in the year 240H.” See Mizaan al-I`tidaal Fi Naqdir-Rijaal (1/233).  Imaam adh-Dhahabee said: “Bishr Ibn Ghiyaath al-Mireesee: A misguided innovator. It is not permitted to narrate from him, and he is given no respect. He learned under Abu Yusuf and then left – and then became proficient in theological rhetoric (`ilmul-kalaam). He then became infamous for saying that the Qur’aan was created, and debating that point. He did not reach the era Jahm Ibn Safwaan [who came before him], but he took his sayings, and used them as proofs and called to them.” See Mizaan al-I`tidaal Fi Naqdir-Rijaal (2/35). A group of the Scholars declared him to be an unbeliever.  Imaam adh-Dhahabee said: “Thumaamah Ibn Ashras Abu Ma`an an-Mumayree al-Basree: From the major scholars of the Mu`tazilah and the heads of misguidance.” Ibid., (2/94).  Al-Baghdaadee said in al-Farq baynal-Firaq (p. 103): “Abu Hudhail Muhammad Ibn Hudhail, well-known as al-`Allaaf. He was the freed slave of [the tribe] Abdul-Qays. He followed the methodology of the offspring of as-Sabaayaa due to the appearance of numerous innovations from them. His corruption knew no bounds, such that all of the sects of the Ummah from his own companions amongst the Mu`tazilah and others declared him to be an unbeliever.”  Al-Haafidh Ibn Hajr (rahimahullaah) said in Lisaan al-Mizaan (6/195): Hishaam Ibn `Amr al-Footee: He was from the companions of Abu Hudhail and a caller to al-I`tizaal.” See also al-Farq bayal-Firaq (p. 145).
Additional notes (translator):
In our times, we see the spreading of the ideologies of Ahlul-Bid’ah across the world due to the proliferation of the internet and various broadcasting tools that are at the disposal of the masses. This has given every misguided innovator an opportunity to propagate his deviation to hundreds and thousands of people.
So upon the person of Sunnah (i.e. the Salafi) is to recognise and distinguish between those who are truthful to this Da’wah and those who are the callers to misguidance, whilst making a claim to be upon the Right Path and the correct Methodology.
The above statement of Imaam al-Barbahaaree and the subsequent explanation of Allaamah al-Fawzaan is a principle of Ahlus-Sunnah – and that is that Ahlul-Bid`ah are to be warned against and not praised. Their callers and scholars are to be shunned and boycotted, both as a protection and fulfilment of textual proofs.
So now you can see how misguided that individual is, who praises the heads of innovation and their callers. How much greater in evil is the one who shares platforms with the innovators and encourages the masses to “benefit” from them.
The words of Al-Barbahaaree are a powerful weapon against the callers to unification between the various sects, and their desire to silence the criticisms and refutations of the Salafis against Ahlul-Bid`ah.
Shaikh Al-Fawzaan shatters the claim of those who say, “I will take the good and leave the evil.” Rather, the callers to bid`ah are not to be “benefitted” from because they will lead you astray, until you eventually start to criticise and attack Ahlus-Sunnah. This is an affair that is seen and experienced in our times – that a person (even a student of knowledge) takes the companionship of one who praises Ahlul-Bid`ah, or sees them to be upon the truth, or sees that one can benefit from them in taking “taking the good and leaving the evil” – and before too long he too begins to criticise Ahlus-Sunnah and distance himself from them and their Scholars. And the end result of that being complete and utter misguidance. There are too many examples of these types of personalities in Europe and North America – some of whom are graduates of the Islamic Universities. They followed their desires, sought worldly positions and found companions who would aid them upon that amongst Ahlul-Bid`ah – so they praise the heads of misguidance whilst finding fault with the Salafiyyeen and their Scholars. The greatest of their deceit is their false claim to Salafiyyah, a term they use as a veil to attract unsuspecting youth to their misguided methodologies. This is why it is so important for the seeker of truth to cling to the Ahlus-Sunnah wal-Jamaa’ah, its scholars, its adherents, their Mosques and their gatherings of knowledge. And also to know the names of those deceitful callers to misguidance in order that they be avoided.
Yet another important benefit in the saying of Imaam al-Barbahaaree (rahimahullaah) is his not be being shy in naming the names of innovators in order to warn others from their misguidance. Warning can be done by describing the characteristics of a person without naming, and by warning against the one who carries those characteristics by name. Here, Imaam al-Barbahaaree (rahimahullaah) names the heads of innovation and Shaikh al-Fawzaan also names them and explains why they are warned against. Furthermore, we see in the footnotes the statements of Imaam adh-Dhahabee and al-Haafidh Ibn Hajar in their books of “criticism and praise” severely criticising these innovators, as was and remains the method of the true Muhadditheen of the past and present. And in actuality, this science of criticising the opposers of the Sunnah has not ceased to remain present amongst the scholars, just as the Prophet (salallaahu `alaihi wassallam) said: “There will never cease to remain a group from my Ummah, manifestly upon the truth. They are unharmed by those who betray them and by those who oppose them, until the command of Allaah comes – and they will remain as such.” (Muslim).