Rarely noticed by general readers, yet quite evident on closer inspection, are the parallels between the phraseology of the Sermon on the Mount in Matthew 5-7 and the vocabulary of the Epistle of James. Over twenty conspicuous similarities can be identified. Various explanations may account for this similarity. Above all, this consistency shows that James and his broad, early Christian audience knew and revered the words of their Lord and Master, especially the directives found in the Sermon on the Mount.
Patrick Hartin, James and the Q Sayings of Jesus, Journal for the Study of the New Testament Supplement Series, vol. 47 (Sheffield: Sheffield Academic Press, 1991), 144-72.