Compare Matthew 6:16 and Didache 8:1

“... The teaching about fasting in Did. 8 is, at one level, not close to teaching about fasting that occurs in Matt. 6. Both texts talk about ‘fasting’ and about the need to be different from the ‘hypocrites’. However, the way in which one is to distinguish oneself from those implicitly attacked is quite different: in Matthew it is via a totally different attitude and manner of fasting, in secret as opposed to publicly; in the Didache it is simply a matter of fasting on different days of the week. For some, this is an indication that the two texts are not directly related to each other at all. On the other hand, the close proximity of this text to the teaching about prayer, and the giving of the Lord’s Prayer in both contexts (cf. the next parallel: it is adjacent in both the Didache and in Matthew) is noteworthy. Further, the talk of one’s opponents as ‘hypocrites’ is very characteristic of Matthew and, one suspects, owes quite a lot to MattR. This is of course not to say that every other reference in Christian literature to ‘hypocrites’ must be dependent on Matthew. Nevertheless, the agreement in language is striking. Moreover, the change in application of the language from Matthew might simply be due to Matthew becoming an ‘authoritative’ text, which, by virtue of being such, lent itself more readily to being reapplied to new situations. Thus the parallel between the Didache and Matthew might be more readily explained if Matthew’s teaching was known in the community of the Didachist and has been reapplied here to a new situation. ...”