Muslim ibn al-Hajjaj
In hadith studies, ibn Isḥaq's hadith (considered separately from his prophetic biography) is generally thought to be "good" ("ḥasan") (assuming an accurate and trustworthy isnad, or chain of transmission) and himself having a reputation of being "sincere" or "trustworthy" ("ṣadūq"). However, a general analysis of his isnads has given him the negative distinction of being a "mudallis", meaning one who did not name his teacher, claiming instead to narrate directly from his teacher's teacher. Because of his tadlīs, many scholars including Muhammad al-Bukhari hardly ever used his narrations in their sahih books. According to al-Khaṭīb al-Baghdādī, all scholars of ahadith except one no longer rely on any of his narrations, although truth is not foreign to him. Others, like Ahmad ibn Hanbal, rejected his narrations on all matters related to fiqh. Al-Dhahabī concluded that the soundness of his narrations regarding ahadith is "hasan", except in hadith where he is the sole transmitter which should probably be considered as "munkar". He added that some Imams mentioned him, including Muslim ibn al-Hajjaj, who cited five of Ibn Ishaq's ahadith in his "Sahih". The muhaddith Ibn 'Adi stated that he didn't find anything which showed any of his hadiths were da'if. He further adds that nothing could stand up to his sirah and maghazi works.