Kiddushin 51A

המרבה במעשר ⁠– פירותיו מתוקנים ומעשרותיו מקולקלין. ואמאי? נימא: כל שאינו בזה אחר זה אפילו בבת אחת אינו.

In the case of one who increases tithes, i.e., he tithes 2/10ths instead of 1/10th of his produce, the remainder of his produce is rendered fit for consumption, as he tithed it properly, but the tithes are ruined, as the additional tithe is neither a tithe nor tithed produce. Since it is unclear which of the 2/10ths is the actual tithe and which is not, the entire 2/10ths may be treated neither as a tithe nor as tithed produce. But according to Rabba’s opinion, why is this produce rendered fit for consumption? Let us say that any matters that cannot be accomplished sequentially cannot be accomplished even if one performs them simultaneously. Since one cannot designate a tithe of 2/10ths sequentially, 1/10th followed by a second tenth, he should be precluded from simultaneously designating 2/10ths of his produce as a tithe. Accordingly, it should be considered as though he had not designated any tithe at all, and his produce should not be regarded as tithed.

אמר ליה: שאני מעשר, דאיתיה לחצאים, דאי אמר: ״תיקדוש פלגא דחיטתא״⁠ – קדשה.

Rabba said to him: The case of tithe is different, as tithe status takes effect partially, i.e., on less than a unit of produce. As, if one said: Let half of each grain of wheat be designated as tithe, he has designated it. Just as one can designate an entire grain of wheat as a tithe, he can likewise designate a half a grain. In this case too, when one tithes 2/10ths of the produce, the ruling is not that 1/10th is the actual tithe and the other tenth is untithed produce. Instead, half of each grain of the designated portion is tithe, while the other half of each grain is not. Accordingly, the remainder of the produce is tithed, as 1/10th of the total has been designated as first tithe. The portion designated as the tithe is ruined, because it is impossible to identify which part of each grain is designated.