Pesachim 51B

ונטל ספיחי כרוב ואכל ונתן לי, ואמר לי: בני, בפני ⁠— אכול, שלא בפני ⁠— לא תאכל. אני שראיתי את רבי שמעון בן יוחי שאכל ⁠— כדי הוא רבי שמעון בן יוחי לסמוך עליו בפניו ושלא בפניו. אתה ⁠— בפני אכול, שלא בפני ⁠— לא תאכל.

and he took cabbage after-growths that had grown during the Sabbatical Year, and ate from them and gave some to me. And he said to me: My son, in my presence, you may eat this. But when you are not in my presence, you may not eat cabbage that grew as an after-growth. I, who saw R' Shimon ben Yoḥai eat, can say that R' Shimon ben Yoḥai is worthy for one to rely upon him both in his presence and not in his presence. You, who did not see him eat, in my presence, rely on what I saw and eat; however, not in my presence, do not rely on my testimony and do not eat. In this case, Rabba bar bar Ḥana maintained that one who saw a Sage act in a certain way may rely on what he saw, as may his students when they are in the presence of their teacher.

מאי רבי שמעון? דתניא: רבי שמעון אומר: כל הספיחים אסורין, חוץ מספיחי כרוב, שאין כיוצא בהן בירק השדה. וחכמים אומרים: כל הספיחין אסורים.

The Talmud asks: What is that statement of R' Shimon? As it was taught in a baraita that R' Shimon says: All after-growths that grow on their own during the Sabbatical Year are prohibited and may not be eaten, except for the after-growths of cabbage, as there is nothing similar to them among the vegetables in the field. The rabbis did not extend the decree prohibiting after-growths to cabbage, because it is unlike other vegetables. Rather, it is like fruit of a tree, which may be eaten if it grows wild during the Sabbatical Year. And the Rabbis say: All after-growths are prohibited, including the after-growths of cabbage.