The non-retaliation statement, “if someone strikes you on your (right) cheek,” from Matthew 5:39-41 and Luke 6:29-30 also appears in Didache 1:4 and in Tatian’s Diatessaron. Dix observes that “a collection ‘Moral Maxims’ ascribed to Isaac of Nineveh’ (Dix 1933, 242), re-works the Two Ways narrative, but takes this passage in Tatian’s reading, which omits the word “right.” The word “right” does appear in Apostolic Constitutions vii, and is also present in the tradition of the Didache which made it through to 1056, the date of the manuscript discovered by Bryennios (Dix 1933, 243). However, the “Moral Maxims,” dating to the seventh century, when apparently quoting the Didache, omits the word. Dix concludes that it would be very unlikely for a scribe or author to omit that word. He also thinks it highly likely that the Didache used the Diatessaron to obtain that unusual reading. This would place the Didache no earlier than 175 A.D.
Dix details a variety of sources for this passage - the Bryennios Didache, the Didascalia in Syriac and in Latin, Apostolic Constitutions vii.2.2, the Georgian Didache, papyrus Ox. 1782, 1., and Isaac of Nineveh (Dix 1933, 244). Dix also brings in Pesudo-Athenasius’ syntagma Doctrinae and St. John Climacus Scala Paradisi as possible sources. The text history is relatively confusing, yet Dix finds it quite possible, though not definitively proven, that the Didache served as a source for many of the others, and that it drew on a collection of sayings which has not been recovered (Dix 1933, 247). Taken together, both the passage about being struck on the cheek and the passage (Did. 1:5a) about lending money suggest significant similarities (Dix 1933, 248). A common source could explain those similarities.