Imaam Ibn Taymiya stated in ‘Majmoo’ il Fataawa’ Vol 3, 283-284:
“If a Muslim is incorrect in his understanding about killing (a Muslim) or making takfir, then he does not become a disbeliever because of that, this is like when Umar Ibn al Khattaab said about Haatib ibn Abi Balta’, “O Rasoolullaah! Allow me to behead this hypocrite!”
Rasoolullaah replied, “Indeed, he is one who witnessed Badr. And you do not know if Allaah has looked towards the people of Badr and said to them ” Do as you like, for you have been forgiven. “ And this [hadeeth] in found in the two Saheeh collections.[ Bukhaari Maghazi 46 & Muslim Fadailu’s-sahaba 161]
And in them is also found the narrations of “al-Ifk”: Usayd ibn Hudayr [ra] said about Sa’d ibn Ubaadah, [ra]; “Indeed, you are a hypocrite, arguing (on behalf of) the hypocrites.” So the two parties (Aws and Hazraj) began to argue, and Rasoolullaah made peace between them.” [Bukhaari Shahadat 15; Magazi 34 & Muslim Tawba 56]
These (Companions mentioned in this hadeeth) were all participants in Badr and from them was the one who said, “You are a hypocrite.”, (and yet) Rasoolullaah did not call him a disbeliever, nor the other, but rather he bore witness that all of them will be in paradise.
And similarly to this is what is authentically established in the two saheeh collections regarding Usaamah ibn Zayd. He killed a man after he said “There is nothing worthy of worship except Allaah”, and Rasoolullaah severly reprimanded him when he heard about it and said to him, ” Oh Usaamah! Did you kill him after he said there is nothing worthy of worship except Allaah? “” I wished that I had not accepted Islam until that day” and he kept on repeating this question to Usaamah until Usaamah [ra] said, [Bukhaari diyat 2 & Muslim Tawba 159]
And along with this, he, Rasoolullaah, did not seek any retribution (from Usaamah) nor blood, nor expiation from him, because he was mistaken (in his belief about killing that man), thinking that that man only said those words to protect himself (from Usaamah’s sword). ”
Imaam Abdul Rahmaan bin Hasan bin Muhammad bin Abdul Wahhab said in (Durar Saniyyah) :
“If it supposed that a person from the Muslims said regarding some people who did matters upon which the scholars have said that it is kufr, while relying upon Quraan and Sunnah in it, while in gheerah for Allaah, and while hating what Allaah hates from these actions, thus it is not permitted for anyone to call in his (the one who made takfir) right (and who makes takfir of a Muslim then he is a kaafir) end, page 132 from the part of Jihaad.