Jamieson, Fausset and Brown on Matthew 10:13

13. And if the house be worthy--showing this by giving you a welcome.

let your peace come upon it--This is best explained by the injunction to the Seventy, "And into whatsoever house ye enter, first say, Peace be to this house" (Lu 10:5). This was the ancient salutation of the East, and it prevails to this day. But from the lips of Christ and His messengers, it means something far higher, both in the gift and the giving of it, than in the current salutation. (See on Joh 14:27).

but if it be not worthy, let your peace return to you--If your peace finds a shut, instead of an open, door in the heart of any household, take it back to yourselves, who know how to value it; and it will taste the sweeter to you for having been offered, even though rejected.