Prayer behind Ahlul bidah

Here are some proofs for your heart's satisfaction:

1. Imam Ibn e Abi Shaibah narrates that Imam Hasan Al-Basri says: The prayer of a Momin which he offers behind a dissembler/hypocrite does not cause him (Momin) any harm, and the prayer of a hypocrite which he offers behind a Momin does not benefit him(the hypocrite) in any way.
(Ibn e Abi Shaybah)

2. Narrated 'Ubaid-Ullah bin Adi bin Khiyar:
I went to 'Uthman bin Affan (May ALLAH be Please with him) while he was besieged, and said to him, "You are the chief of all Muslims in general and you see what has befallen you. We are led in the Salat (prayer) by a leader of Al-Fitan (trials and afflictions etc.) and we are afraid of being sinful in following him."
'Uthman (May ALLAH be Please with him) said. "As-Salat (the prayers) is the best of all deeds so when the people do good deeds do the same with them and when they do bad deeds, avoid those bad deeds.
(Bukhari)

3. The Prophet (SALL ALLAHU 'ALAHI WA AALIHI WA SALLAM) said: Striving in the path of Allah (jihad) is incumbent on you along with every ruler, whether he is pious or impious; the prayer is obligatory on you behind every believer, pious or impious, even if he commits grave sins; the (funeral) prayer is incumbent upon every Muslim, pious and impious, even if he commits major sins.
(Abu Dawud and Bayhaqi)

4. Ja'far Bin Burqan narrates that he asked Memoon Bin Mehran (Tabi’) about (offering Prayer behind) a person to which he replied:
You offer Prayer for ALLAH and not for him,(You ask about him) whereas we used to offer Prayer behind Hujjaj (Bin Yousuf) who was a Haroori Arzaqi (Kharji).
(Ibn e Abi Shaybah)

5. Nafi’ says that it was asked to Ibn e Umar (R.A) during the time of Ibn e Zubair (R.A) and Khawarij that “you offer Prayer with them” (In Bayhaqi’s narration it is mentioned as “you take their Salam”) and (“you offer Prayer”) with Ibn e Zubair (R.A) and sometimes with Khawarij too while they (the Khawarij) murder other Muslims.
To this, Ibn e Umar (R.A) said:
I respond to those who say “come towards Prayer” (meaning I accept their call), and I (also) respond to those who say “Come towards success”. And those who say “Come, let’s kill our Muslim brothers and burglarize their property”, I turn down/refuse their call.
(Bayhaqi and Abu Nu'ym)

6. Narrated by Abdullah Bin Mas’ud (Radi ALLAH Ta’ala Anhu) that PROPHET (Peace be Upon him) said: Three things are from Sunnah (and one of them is) Praying behind Every Imam. You will have the reward of your Prayer and his sins will be on him.
(Dar Al Qutuni)

7. Abdullah Bin Umar R.A narrates that Nabi (SALL ALLAHU 'ALAHI WA AALIHI WA SALLAM) said:
Offer the funeral prayer for each of that person who says LA ILAAHA ILLA ALLAH and offer prayer behind each of that person who says LA ILAAHA ILLA ALLAH.
(Tabrani, Imam Dar Al Qutuni , Imam Tamam Bin Muhammad and Imam Abu Nu’ym)