משנה על פסחים 3:1

One who wishes to make his field into square plots [each sown] with a different species, he should divide it into twenty-four square plots for a bet seah, a square plot per bet rova, and he may then sow in each whatever species he wants. If there is one square plot or two [inside his field], he may sow them with mustard, but if there are three he may not sow them with mustard, since it looks like a field of mustard, the words of Rabbi Meir. But the sages say: nine square plots are permitted, ten are forbidden. Rabbi Eliezer ben Yaakov says: even though the whole of his field is a bet-kor, he may not make within it more than one square plot.