Leviticus 27:31

ממעשרו [AND IF A MAN WILL REDEEM] OUGHT OF HIS TITHES [HE SHALL ADD THERETO ITS FIFTH] — of his tithes, but not of the tithes of his fellow: if he redeems the tithe of his fellow he need not add its fifth (Kiddushin 24a). Wherein consists the redemption of the “second tithe” (since it is permitted as food to the owner)? He redeems it in so far as he makes it permissible for food at any place (which is not the case if he does not redeem it) and the redemption-money he brings up to Jerusalem, buys eatables and eats them there as it is said, (Deuteronomy 14:25, 26) “Then thou shalt put into money… [and thou shalt lay out that money… for cattle, for sheep etc.]”.

וְאִם־גָּאֹ֥ל יִגְאַ֛ל אִ֖ישׁ מִמַּֽעַשְׂר֑וֹ חֲמִשִׁית֖וֹ יֹסֵ֥ף עָלָֽיו׃

And if a man will at all redeem of his tithes, he shall add to it the fifth part of it.