Imam Ahmad said:

A ‘Adl (i.e. Trustworthy Reliable [narrator]) has to have six qualities: a Faqeeh (i.e. knowledgable in Fiqh), a Scholar, a Zaahid (i.e. Ascetic), Pious, Chaste, Insightful (or having a sharp eye) of what to set forth [and accept of narrations], and Insightful about what to leave.


[1] Al-Maqsad Al-Arshad (3/164)

قال الإمام أحمد: ينبغي للعدل أن يكون فيه ست خصال: فقيها, عالما, زاهدا, ورعا, عفيفا, بصيرا بما يأتي, يصيرا بما يذر.ـ