Shevi'is, Chapter 10, Mishnah 8. This mishnah deals with a borrower who wishes to repay a debt after the Shemittah year has passed, and how the lender is to conduct himself in such a case.
"HaMachzir chov ba'shevi'is":
Once Shemittah has passed, the loan has been cancelled and the lender may not claim it; the borrower no longer owes the money. Even so, the borrower comes and wishes to repay the debt of his own accord. In such a case the lender must say to him explicitly: "Meshamet ani" - meaning, I release the debt and permit you not to pay.
If the borrower insists and says: "Af al pi chen" - nevertheless I wish to repay you - "yekabel mimenu", and the lender is permitted to accept the money.
And the one statement that he made is enough, as it says: "V'zeh devar ha'shemittah". From here they derived a hint that one word of Shemittah suffices - a single statement that he is not claiming the loan and is prepared to forgo it.
Similarly - a killer who was exiled to a city of refuge:
Along the lines of this principle that one statement suffices, the mishnah brings an entirely separate halachah: an unintentional killer, who killed without intent and was obligated to go out to a city of refuge, and went there. If the people of the city wished to honor him, he says to them: "Rotze'ach ani" - meaning, know that I am not worthy of honor, for I must be sorrowful and do teshuvah over the fact that such a terrible thing was done through me.
If the people of the city said to him: "Af al pi chen" - even though we know that you are a killer, we wish to honor you - "yekabel mehem", and he is permitted to accept the honor. As it says: "V'zeh devar ha'rotze'ach" - one statement suffices, and afterward he may accept the honor.
In summary: in this mishnah we learned two halachos built on a single principle - one statement is enough: the lender says to the borrower "Meshamet ani", and when the borrower insists he may accept from him; and the killer who was exiled says to the people of the city "Rotze'ach ani", and when they insist he may accept the honor from them. Both laws were derived from the wording of the verse: "V'zeh devar ha'shemittah" and "V'zeh devar ha'rotze'ach".