The default position concerning the report of a heretic...

The default position concerning the report of a heretic, if he was accurate and trustworthy, is to accept it, regardless of whether he narrated concerning what agrees with his bid’ah (heresy) or not , as long as he had not apostatized through his heresy. In such a case, it will be rejected due to his kufr (disbelief).

This was the practice of the Imams who were hadith scientists, for they used to narrate from the heretic if he was trustworthy and accurate, and used to declare his report to be sahih . For verily, Imam Ahmad has recorded in his Musnad , and Muslim in his Sahih , and al-Nasai in al-Kubra and al-Mujtaba , and al-Tirmidhi and Ibn Majah, and Ibn Hibban in his Sahih , and Ibn Mandah in Kitab…