Ma’aser A’ni – The Poor Man’s Tithe: Where Does the Torah Specify The Third and Sixth Year?

Parshas Re’eh:
At the end of three years you shall bring forth all the tithe of your produce in that year, and shall lay it up inside your gates; And the Levite, because he has no part nor inheritance with you, and the stranger, and the orphan, and the widow, who are inside your gates, shall come, and shall eat and be satisfied; that the Lord your God may bless you in all the work of your hand which you do.
--Devarim 14:28-29

The Rambam (Sefer haMitzvos, Aseh 130) says we are commanded to set aside the poor man’s tithe in the third year and sixth year i.e. three years later means every three years, of every Shemittah cycle. This is contained in His words: At the end of three years you shall bring forth all the tithe of your produce in that year….

When learning this mitzvah in the shiur Reb Yitzchak Klein asked: Where does the Torah specify the third and sixth Year?

Reb Labe Marcus pointed to the Ibn Ezra who says: At the end of three years – that is ma’aser shlishi and ma’aser sheni is not taken in that year….

The Torah says further on in Parsha Ki Savo more explicitly:
When you have finished tithing all the tithes of your produce the third year, which is the year of tithing, and have given it to the Levite(ma’aser rishon), the stranger, the orphan, and the widow(ma’aser a’ni, that they may eat inside your gates, and be filled.
--Devarim 26:12

The Gemara says:
R. Joshua b. Levi says: [It is written], When thou hast made an end of tithing all the tithe of thine increase in the third year, which is the year of the tithe. This means the year in which there is only one tithe. How is then one to act? [He gives] the first tithe and the tithe of the poor, and the second tithe is omitted. Is this correct, or should the first tithe also be omitted? — [Not so], because it says, Moreover thou shalt speak unto the Levites and say unto them, When ye take of the children of Israel the tithe which I have given you from them for your inheritance. The text here compares the tithe [of the Levites] to an inheritance, [to signify that] just as an inheritance is to be held uninterruptedly, so their tithe is to be given without interruption. It has been taught to the same effect: ‘When thou hast made an end of tithing etc.’ [This means] a year in which there is only one tithe. How is one to act? [He gives] first tithe and tithe of the poor, and the second tithe is omitted. Should perhaps the first tithe also be omitted? — [Not so], because it says, and the Levite shall come, which means to say, every time he comes give him. So R. Judah. R. Eliezer b. Jacob says: We have no need [to appeal to this text]. It says, Moreover thou shalt speak unto the Levites and say unto them, When ye take from the children of Israel the tithe which I have given you from them for your inheritance. The text here compares the tithe to an inheritance, to signify that just as an inheritance is held uninterruptedly, so the tithe is to be given without interruption.
--Rosh Hashanah 12b (See also Rambam, Matnos A’niyim 6:4)