Maseches Peah, Chapter 4, Mishnah 8. In the previous Mishnah we learned the principle that hekdesh is exempt: produce belonging to hekdesh is not obligated, provided that it belonged to hekdesh at the moment the obligation took effect. Our Mishnah deals with this same principle, only this time with regard to the obligation of Terumah and tithes: this obligation does not apply to produce belonging to hekdesh, and here too it depends on whether the produce belonged to hekdesh at the moment the obligation took effect - the time the Mishnah calls 'onas hama'asros'.
The words of the Mishnah:
"Kayotzei bo" - similar to the previous Mishnah, which discussed hekdesh, that it is exempt from leket, shich'chah and peah.
"Hamakdish peirosav ad shelo ba'u le'onas hama'asros ufda'an" - one who dedicates the produce of his crop before it reached the stage of onas hama'asros, which is the time when it becomes obligated in tithes, and he also redeemed it before the obligation took effect - it is obligated, since the obligation of tithes had not yet taken effect while it belonged to hekdesh. The mere fact that it was once hekdesh is not enough to exempt it.
"Mishe'ba onas hama'asros... ufda'an" - after the stage at which the obligation of tithes takes effect had already passed, he dedicated the produce and afterward redeemed it - it is obligated, because at the time of onas hama'asros it did not belong to hekdesh.
"Vehikdishan ad shelo nigmeru vegamar hagizbar ve'achar kach pada'an" - he dedicated the produce before its completion, which is what finalizes the obligation of tithes, and the one who completed the work was the treasurer, the official in charge of hekdesh. It turns out, then, that at the moment the obligation of tithes took effect the produce belonged to hekdesh, and therefore even after he redeemed it, it is exempt from Terumah and tithes. The reason is stated explicitly in the Mishnah: "Shebish'as kevi'asan hayu peturin" - at the time their obligation was fixed they were exempt, because they belonged to hekdesh.
What is 'onas hama'asros'?
The commentators disagree about the definition of the term 'onas hama'asros' used by our Mishnah, since Maseches Ma'asros describes two different times at which the obligation takes effect:
The stage of ripening: once the produce reaches a certain stage of ripening, it becomes fit for the first time for the separation of Terumah and tithes and is considered tevel. From that point on a person may not eat a fixed meal from it until he separates Terumah and tithes from it, but casual eating, in the manner of nibbling, is still permitted.
The completion of the work: a later point, after the produce has grown to its full measure and been harvested, and after a certain stage of the processing that the farmer does to it for storage and the like. Once it reaches one of these points, depending on the type of produce, this is the second time at which the obligation of tithes takes effect, and from this stage even casual eating is forbidden, and a person may not eat from it at all.
Now we must clarify: which of these two times of the tithe obligation does our Mishnah mean?