Demai, Chapter 4, Mishnah 7. This mishnah deals with a city that depends on the donkey drivers who come to it and supply its residents with the produce they need, and the question it addresses is whether their statements can be relied upon or not.
A donkey driver who praises his fellow's produce:
Donkey drivers bring grain and enter the city, and one of them - who is not known to the local people - testifies in praise of his fellow's produce in one of two ways:
"Sheli chadash veshel chaveri yashan" - my produce is newer, while my fellow's is older and of better quality. The advantage of the older grain is as we saw in the previous mishnah: halachically it may be eaten even before the Omer offering has been brought; alternatively, in terms of quality, it is drier and therefore superior.
"Sheli adayin lo nit'lah mimenu tumas meis, veshel chaveri nit'lah mimenu" - here too he presents his own produce as inferior to that of his fellow.
In both cases the donkey driver praises his fellow at his own expense, and nevertheless the mishnah rules: "Einam ne'emanim" - they are not believed.
Why are they not believed?
Seemingly he has no reason to do his fellow a favor, so why should he not be believed? The reason is the concern for a collusion arranged between the donkey drivers: in one city this one will praise his fellow, and in the other city his fellow will praise him. Because of the concern for such an arrangement between them, they are not believed.
The view of Rabbi Yehudah:
Rabbi Yehudah disagrees and holds that they are believed. In his opinion, the leniency they were granted was instituted for the sake of the food supply of the city's residents, and therefore their statements may be relied upon, and there is no need to be concerned that they made a deal between them to do the reverse in the other city.
To summarize: In this mishnah we learned about donkey drivers who enter a city, one of whom praises his fellow's produce over his own - whether by claiming that his is new and his fellow's is old, or by claiming that his fellow's has had tumas meis removed from it. The Sages do not believe him, out of concern for a mutual collusion between the donkey drivers, while Rabbi Yehudah accepts them and believes them because of the food supply needs of the city's residents.