Ibn Qudaamah al-Maqdisee says in his book Lum'ah al-I'tiqaad:
And from the Sunnah is abandoning the People of Bid'ah, opposing them, leaving arguing and disputing in the religion, avoiding looking into the books of bid'ah and listening to their speech, and every newly-invented matter into the religion is a bid'ah.
Ibn Uthaymeen says in his explanation of this book:
Al-Hijraan (Abandoning) is the verbal noun of the verb Hajara (to abandon, forsake, leave, etc...). Linguistically, it means to leave or abandon and what is meant by abandoning the People of Bid'ah: avoiding them, not loving them, befriending or supporting them, not saying Salaam to them, visiting them, nor visiting them when they are sick, etc.
Abandoning the People of Bid'ah is Waajib due to Allaah's statement:
"You will not find a people believing in Allaah and the last day befriending those who oppose Allaah and His Messenger." [Al-Mujaadilah : 22]
Also because the Prophet (Sallallaahu 'Alaihi Wa Sallam) made this Hijraan from Ka'b ibn Maalik and his two companions when they failed to participate in the Battle of Tabook. 
However, if there is a benefit in sitting with them in order to clarify the truth to them and to warn them from bid'ah  , then there is no harm in that and this may be required from the statement of Allaah:
"Call to the way of your Lord with wisdom and good preaching and dispute with them in the best manner." [An-Nahl : 125]
This could possibly be by sitting with them, verbally, or perhaps by exchanging letters or writings. Whoever abandons the People of Bid'ah, then he should leave off looking into their books, fearing from a fitnah that could result from them. (He should also leave off) circulating them (their books) among the people. So avoiding the sources of misguidance is Waajib from the statement of the Prophet (Sallallaahu 'Alaihi Wa Sallam) regarding the Dajjaal:
"Whoever hears of him, then he should avoid him. For by Allaah, indeed a man will come to him thinking himself to be a believer and yet will follow him due to the doubts which surround him." Aboo Daawood recorded it. Al-Albaanee said, "And its chain is Saheeh." [ Ahmad (4/43, 441), Aboo Daawood (4319), Al-Haakim (4/531). Declared Saheeh by Al-Albaanee in Saheeh Al-Jaami' (6301) ].
However, if the goal behind looking into their books is to become aware of their bid'ahs for the purpose of refuting them, then there is no harm in that for the one who has the correct 'Aqeedah with which he can protect himself with it, being capable of refuting them. Rather, perhaps this is even Waajib because refuting bid'ah is Waajib and whatever mean is necessary to fulfill something that is Waajib, then the mean itself is Waajib.
 The story of Ka'b ibn Maalik's repentance and his two companions is found in Al-Bukhaaree (4418) and Muslim (2769)
 And this is determined by the scholars aware of the situation.
Source: Lum'ah al-I'tiqaad of Imaam Ibn Qudaamah al-Maqdisee
Translator: Abuz-Zubayr 'Abdur-Rahmaan (Scott) Harrison - Rahimahullaah
Note: As salaamu 'alaykum wa Rahmatullaahi wa Barakaatuh... Our brother was recently taken by the Will of our Lord Ta'aalaa, he was our brother in Deen and Salafiyyah and a dear friend, a father and husband. He has helped our growth in the da'wah for a number of years translating our articles and books. May Allaah forgive him of his sins, make these deeds grand on his scale of good deeds, and make him from the occupants of the highest level of paradise, Aameen yaa Rabbanaa! From what we knew of him, he was truly from the best of people and Allaah knows best... Inna lillaahi wa Inna Ilayhi Raaji'oon.
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