The 128th mitzvah is that we are commanded to separate ma'aser sheni.1
The source of this commandment is G‑d's statement,2 "Take a tithe [Aser t'aser] from year to year of all the seed crops that come forth in the field."
The Sifra says: "The words 'Shanah shanah' ['from year to year'] teach us that one may not take ma'aser from one year's crops for [what should have been taken from] another year's crops. But this only tells us about the subject of this verse, ma'aser sheni.3 How do we know that this applies to the other types of ma'aser? From the words, Aser t'aser."4
The Torah says explicitly5 that ma'aser sheni is brought to Jerusalem and eaten there by its owners, and we have already mentioned6 the words of our Sages on this subject.
The verse itself also gives details of this mitzvah: that when it is impossible to actually bring it because of the distance, it must be redeemed, with the money brought to the Bais Hamikdosh and spent on food. The source of this law is G‑d's statement,7 "If the place is too far for you and you cannot carry it there." The Torah has given other details of this mitzvah: that if a person redeems it, he must add one-fifth.8 The source of this is G‑d's statement,9 "If a person wishes to redeem his tithes, he must add an additional fifth."
The details of this mitzvah are explained in tractate Ma'aser Sheni.
This too is a Biblical mitzvah only for produce which grew in Eretz Yisroel, and may be eaten only when the Bais Hamikdosh is standing. In the words of the Sifri: "The verse10 compares eating a b'chor [first-born animal] and ma'aser sheni: just as the b'chor may be eaten only when the Bais Hamikdosh is standing, so too ma'aser sheni may be eaten only when the Bais Hamikdosh is standing."
