Shabbat 17B

טבי רישבא אמר שמואל: אף גידולי תרומה תרומה, בו ביום גזרו. מאי טעמא? אמר רבי חנינא: גזירה משום תרומה טהורה ביד ישראל.

Tavi the bird hunter [rishba] that Shmuel said: The decree that growths of teruma, i.e., produce that grows from teruma that was planted in the ground, are considered teruma, the rabbis also issued on that day. The Talmud asks: What is the reason for this decree? R' Ḥanina said: A decree due to pure teruma in the hand of a non-priest Israelite. One who seeks to avoid giving teruma to a priest would plant it in the ground and thereby negate its teruma status. To prevent him from doing so, the rabbis decreed that that which grows from the teruma is also considered teruma. Consequently, one would gain nothing by replanting the teruma.

אמר רבא: אי דחשידי להכי, אפרושי נמי לא ליפרשו! (אלא אמר רבא: ישראל) כיון דאפשר למעבד חטה אחת כדשמואל ולא קעביד, הימוני מהימני. אלא גזירה משום תרומה טמאה ביד כהן, דילמא משהי לה גביה ואתי לידי תקלה.

Rava said: If they are suspected of that, let them refrain from separating teruma altogether. Rather, Rava said: We know that with regard to an Israelite, as opposed to a Levite, fundamentally it is possible to perform the mitzva of teruma by separating merely one grain of wheat, in accordance with the opinion of Shmuel, who said that by Torah law there is no fixed measure for teruma. By separating one grain of wheat as teruma for all the wheat on the threshing floor, one fulfills his obligation. Since he nevertheless did not take advantage of that possibility to exempt himself from the obligation of separating teruma, he is trustworthy, and there is no reason to suspect that he will seek to avoid giving teruma to the priest by planting it. Rather, the reason for the decree is due to impure teruma in the hand of a priest. A priest is forbidden to eat impure teruma and he is required to burn it. However, the priest is permitted to derive benefit from its burning. The rabbis were concerned lest he keep the impure teruma with him until the season of sowing and sow his field with it, and, as a result, he encounter a stumbling-block because over time he is liable to forget that the teruma is impure and eat it.