Take and offer at my command, and for my service, an heifer of three years old, at which time it is perfect in stature and strength, and therefore fittest for God's service. This and the other creatures here following, and sacrifices, are the same which afterwards were prescribed in the Levitical law.
Related Commentaries of Genesis 15:9
Genesis 15:9
9 And he said unto him, Take me an heifer of three years old, and a she goat of three years old, and a ram of three years old, and a turtledove, and a young pigeon.