Genèse 4:3

And it was at the end of days, on the fourteenth of Nisan, that Kain brought of the produce of the earth, the seed of cotton (or line), an oblation of first things before the Lord;

AND CAIN BROUGHT OF THE FRUIT OF THE GROUND AN OFFERING UNTO THE ETERNAL. 4. AND ABEL, HE ALSO BROUGHT. These men [Cain and Abel] understood the great secret of the sacrifices and the meal-offerings. So also did Noah, [who likewise offered sacrifices]. Our Rabbis have said that the first man also sacrificed a bullock. This should close the mouth of those who speak foolishness concerning the reason of the sacrifices. I will yet intimate a great principle concerning this matter with the will of the Holy One, blessed be He.

מנחה, a gift of sorts, as in Exodus 32,34. The word is derived from the root נחה as in נחה את העם. [understanding the word נחה in the imperative mode as related to presenting a gift is certainly novel. Ed.]

ויהי מקץ ימים, it was some time after Kayin had become a farmer tilling the soil, that he offered a gift to G’d. The Torah is deliberately vague about how much time had elapsed, writing only מקץ ימים, at the end of a number of years. It is reasonable to assume that the word ימים refers to years rather than days, as it is clear from Leviticus 25,29 that in that verse the word ימים represents the number of days that make up a year. It would seem therefore, that after having worked the soil for a full year Kayin offered a gift to G’d, part of what he had harvested. He did this in the spot in which his father Adam had offered prayer and had offered a sacrifice of his own as an acknowledgement of his gratitude, his sons presumably having been in attendance. Although the Torah does not spell out the composition of Adam’s sacrifice, the Talmud in Shabbat 28 suggests that it included a one-horned ox, basing itself on Psalms 69,32 משור פר מקרן מפריס, “ox with horn and hooves,” His sons imitated him, each one bringing a gift from what his labour had produced. The offering was meant to be an expression of man’s gratitude to G’d for the success of his endeavours.
מפרי, the Torah is vague about precisely what the offering consisted of, other than saying that it was part of the fruit produced by the earth. We know nothing about the quality or quantity of this offering. Seeing that when describing Hevel’s offering the Torah adds the words: “from the firstborn of his sheep and from their best ones,” it is reasonable to assume that Kayin’s gift, by comparison, was stingy, in fact represented an insult to G’d rather than a grateful acknowledgment of G’d’s part in making the earth produce a crop for him. This is why his offering was not welcome, was rejected. Had Kayin at least brought of the best quality of the produce he had grown, the Torah would have mentioned this to his credit, just as it had done in the case of Hevel’s offering. As a result of the philosophical contrast expressed by these two offerings, i.e. Kayin’s flax and Hevel’s wool, mixing these two fibres in one garment has been outlawed by the Torah for the Jewish people. The lesson for us is that the gifts proffered by a Kayin and the gifts proffered by a Torah-observant Jew are not compatible with one another.