Sheviis, Chapter 5, Mishnah 5. The Mishnah addresses a question: from what point after the Sheviis year has ended is one permitted to buy luf, which is a type of onion? The question concerns buying from someone who is suspect, one who is suspected of not observing the laws of Shemittah: from what point may we assume that the produce in his possession is not Sheviis produce but grew afterward, and how long after Shemittah ends may the buyer rely on that assumption?
Two opinions are recorded in the Mishnah:
"Rabbi Yehudah omer: miyad" - one may buy immediately once Sheviis has ended. His reasoning: if someone has luf for sale after Shemittah has ended, in most cases the luf was already in his possession from before Shemittah, whether it was still in the ground, or out in the field, or stored beneath the earth. This is not something that came into being during Sheviis, but rather produce that preceded it.
"VaChachamim omrim: mishe'yirbeh he'chadash" - one may not buy until new luf has already grown in a normal quantity, so that most of what is being sold comes from the new crop. From that point on, one may buy even from someone who is suspect regarding the laws of Shemittah. Before then, however, it is forbidden, because there is reason to suspect that even if the seller claims that the luf in his possession is from the pre-Shemittah crop, perhaps his words are not true.
In summary: the Mishnah records a dispute as to when one may buy luf from a suspect person after Sheviis: according to Rabbi Yehudah, immediately, because most luf offered for sale originates from before Shemittah; and according to the Sages, only "mishe'yirbeh he'chadash", out of concern that the seller's words are not true.