All of the Sahabah are trustworthy ('udul sin:adl). That is why people did not ask about the isnād in the time of the Prophet  and after him  until the fitna (civil war) took place then they asked about the isnād. The Sahabah and others (after them) encouraged people to examine the one from whom the hadīth is taken. It has been narrated by Abu Sakina Majashi' ibn Fateena that he said: I heard 'Ali ibn Abu Talib  that he was in the mosque of Kufa where he said:

Al-Dahhak ibn Muzahim said:

After the fitna (civil war), several Islamic sects arised which adopted novel opinions. The followers of these sects claimed that they deduced these opinions, which they came to profess, from the Sharī’ah texts until they became Islamic opinions. And when some of them required a proof but did not find the evidence in the Sharī’ah texts for the opinion he holds then he would fabricate a hadīth which supported his opinion and he would attribute it to the Messenger . Some of them use to carry the call to join his sect and grow interest in it or call (Da’wah) to their opinions and desire to fabricate the ahadīth. These new opinions were termed as bid'as (innovations) and the people who did this were called mubtadi'a (innovators). This is why taking ahadīth from these people is subject to scrutiny and their narration of hadīth used to be a subject of debate. There are detailed clarifications regarding their situation. Thus the mubtadi' (innovator) who is charged with kufr due to his bid'a (innovation), there is no problem in rejecting his narration. If he is not charged with kufr but he permits lying then his narration is rejected as well. As for the ones who does not permit lying then his narration is accepted on the provision that he does not call (invite) to his sect or school (mazhab). If he calls to his sect then his narration is rejected and his reports are not advanced as proofs.

In short, any Muslim who meets the conditions for the acceptance of a narration, if he is trustworthy ('adl) and accurate (dabit) then his narration is accepted irrespective of his mazhab or sect as long as he does not call to his sect or mazhab because inviting people to the sect or mazhab is not allowed. As for the one who invited people to Islam and explained the thoughts he has adopted with their evidences then his narration is accepted because then he is calling people to Islam. And this person's narration is not impugned.