Genesis 15:12

And when the sun was going down, a deep sleep fell upon Abram; and, lo, an horror of great darkness fell upon him. At the direction of God, Abram prepared the sacrificial animals, the heifer, the ram, and the goat being cut in half lengthwise and the several pieces laid over against one another, by pairs, while the birds were not divided. The entire proceeding filled Abram with the deepest awe. When the birds of prey, eagles and vultures, came down to feed on the carcasses, Abram turned them away, drove them off. The ceremonial which was here enacted was that in use from ancient times, the contracting parties passing between the halves of the dead animals to indicate their reconciliation to a unity. The culmination of the strange scene came when, as the sun was setting, Abram was overcome by a deep sleep and a great horror fell upon him.