In the Apocalypse of Abraham, Abraham receives a divine command to offer sacrifice1:
IX. Then a voice came to me speaking twice: “Abraham, Abraham!” And I said: “Here am I!” And He said: “Behold, it is I; fear not, for I am before the worlds, and a mighty God who hath created the light of the world. I am a shield over thee, and I am thy helper. Go, take me a young 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 pigeon, and bring me a pure sacrifice. And in this sacrifice I will lay before thee the ages (to come), and make known to thee what is reserved, and thou shalt see great things which thou hast not seen (hitherto); because thou hast loved to search me out, and I have named thee my Friend. But abstain from every form of food that proceedeth out of the fire, and from the drinking of wine, and from anointing (thyself) with oil, forty days,” and then set forth for me the sacrifice which I have commanded thee, in the place which I will shew thee, on a high mountain, and there I will shew thee the ages which have been created and established, made and renewed, by my Word, and I will make known to thee what shall come to pass in them on those who have done evil and (practised) righteousness in the generation of men.
What is the reason for the specificity of the command? Why these animals, and why do their ages matter? Why fast for forty days?
1 The Apocalypse of Abraham. Edited, With a Translation From the Slavonic Text and Notes by George Herbert Box.