Young, Stephen E. "Chapter Eight: Three Isolated Sayings from the Jesus Tradition." Jesus Tradition in the Apostolic Fathers. Tübingen: Mohr Siebeck, 2011, 226-238.
In this chapter, Young reviews three isolated sayings attributed to Jesus, as opposed to his previous analysis of a cluster of statements or a chunk of liturgy. One significant challenge at this point is the fact that two of the sayings are rather proverbial. Young considers it unlikely that an author would consult a written source to deal wit ha proverb (Young 2011, 226).
Didache 9.5b is identical to Matthew 7:6. The first part of Didache 9.5 introduces it as something the Lord said. Young considers it more likely that both authors drew on an oral tradition than finding a literary relationship (Young 2011, 227).
Ignatius, in his Letter to the Smyrneans, 3.2a has Jesus telling his disciples to touch him and see that he is not a spirit. The same command in the same setting is recorded in Luke 24:39 (Young 2011, 229). Young notes that Jerome attributed Ignatius' source as the Gospel according to the Hebrews. However, Jerome made this assessment based on Eusebius' Ecclesiastical History 3.36.11, which denies knowing where the saying originated (Young 2011, 230). Young finds little evidence that Ignatius used Luke. Further, the use of terms is not very similar. Young concludes it is likely tht Luke and Ignatius depended on a common source (Young 2011, 231).
Finally, Young considers Polycarp's Epistle to the Philippians 7.2c, where the Lord is cited as saying, "For the spirit is willing but theflesh is weak." There is verbatim agreement with Mark 14:38 and Matthew 26:41 (Young 2011, 232). The context of the statement differs in Polycarp, suggesting that the source would havebeen lacking in context. However, Polycarp does have part of the context, which suggests to Young that there may have been a collection of sayings which were grouped together somehow (Young 2011, 234). The periodic structure and the aorist εἶπεν suggest an oral source to Young (Young 2011, 236).