הלכה על קידושין 47:2

The commandment of the second tithe: To remove the second tithe from the produce in four years of the sabbatical cycle, meaning to say, after we separate the first tithe that is given to the Levites, that we separate yet another tithe. And hence it is called the second tithe. And the law of this tithe is that it be eaten in Jerusalem. And about it is it stated (Deuteronomy 14:22), "You shall surely tithe the produce of your seed." And Scripture elucidates that if the place is far from us and we cannot carry it there except with great burden and much expense, that we can redeem it and bring up its value [to] Jerusalem and spend it there only for the needs of eating and drinking. And Scripture likewise elucidates that the one who redeems his tithe needs to add a fifth to the value - which is to say that if it was worth four dinar, that he eat instead of it [that which costs him] five dinar in Jerusalem. And about this is it stated (Leviticus 27:31), "And if a man surely redeems from his tithe, he shall add its fifth to it." And they, may their memory be blessed, made a precise inference (Kiddushin 24a): "'From his tithe - but not from the tithe of his fellow; 'a man from his tithe' - but not a woman."