Midrasch zu Tehillim 36:5

Rabbi Simeon said: The Holy One, blessed be He, pointed out the altar with a finger to Abraham our father, and said to him: This is the altar. That was the altar whereon Cain and Abel sacrificed; it was the same altar whereon Noah and his sons sacrificed, as it is said, "And Abraham built the altar there" (Gen. 22:9). "And Abraham built an altar there" is not written here, but "And Abraham built the altar there." That was the altar whereon the first ones (of old) had sacrificed.