Genesis 15:9

Take me an heifer — Perhaps Abram expected some sign from heaven, but God gives him a sign upon a sacrifice. Those that would receive the assurances of God's favour, must attend instituted ordinances, and expect to meet with God in them. Observe, God appointed that each of the beasts used for his service should be three years old, because then they were at their full growth and strength. God must be served with the best we have. We do not read that God gave Abram particular directions how to manage these, knowing that he was well versed in the custom of sacrifices. Abram took as God appointed him, though as yet he knew not how these things should become a sign to him. He divided the beasts in the midst, according to the ceremony used in continuing covenants, Jeremiah 34:18, where it is said, they cut the calf in twain, and passed between the parts. Abram, having prepared according to God's appointment, set himself to expect what sign God would give him by these.