(Bamidbar 18:26) "And to the Levites shall you speak, and you shall say to them: When you take from the children of Israel the tithe (ma'aser) that I have given to you from their inheritance, (then you shall separate from it the terumah of the L-rd, ma'aser from the ma'aser.") Why is this written? To teach that (Ibid. 21) "And to the sons of Levi, behold, I have given every tithe" speaks of (one-tenth of) the produce (of the land, and not of [one-tenth of] the land itself.)
You say, the produce, but perhaps (the reference is to) the land (itself)! It is, therefore, written (26) "And to the Levites you shall speak and you shall say to them (… that I have given to you from them in their inheritance"). "that I have given to you from them in their inheritance": Because they have not been given a portion in the land, there has been given to them one-tenth of the produce. "then you shall separate from it": From one kind (of produce) for its kind, and not from one kind for a different kind, and not from what is rooted for what is unrooted, and not from what is unrooted for what is rooted, and not from the new (crop) for the old, and not from the old for the new.
And whence is it derived that one is not to take terumah from produce of the land (Eretz Yisrael) for produce outside the land or from produce outside the land for produce of the land? From (Vayikra 27:30) "And all the tithe of the land, etc.)" Variantly: "from it": This is "extra" (mufneh) for formulating an identity (gezeirah shavah) viz.: It is written here "from it," and, in respect to the Paschal lamb, (Shemot 12:9) ("Do not eat) from it, etc." Just as re "with it" mentioned here (in respect to ma'aser), it (ma'aser) is forbidden to a mourner, (viz. Devarim 26:14), so, re "with it" mentioned in respect to Pesach, it (the Paschal lamb) is forbidden to a mourner.