Genesis 4:4

And Abel, he also brought of the firstlings of his flock and of the fat thereof. In the very mention of the gift there is an indication of the difference in the attitude of the hearts; for whereas it is said of Cain only in general that he brought of the fruit of the soil, it is stated concerning Abel that he brought of the first-born of his flock, such as were in the best of condition, rich in fat. The gifts thus expressed the difference between Abel's free and joyful faith and Cain's legal, reluctant state of heart, Hebrews 11:4; 1 John 3:12. And the Lord had respect unto Abel and to his offering;