Q source
The content of James is directly parallel, in many instances, to sayings of Jesus which are found in the gospels of Luke and Matthew, i.e., those attributed to the hypothetical Q source. Compare, e.g., "Do not swear at all, either by heaven...or by the earth...Let your word be 'Yes, Yes' or 'No, No'; anything more than this comes from the evil one" and "...do not swear either by heaven or by earth or by any other oath, but let your 'Yes' be yes and your 'No' be no, so that you may not fall under condemnation" (James 5:12). According to James Tabor, the epistle of James contains "no fewer than thirty direct references, echoes, and allusions to the teachings of Jesus found in the Q source."