I said it was first preached in Genesis. Though I'm sure that's why that choice was there, I will post the scriptures of support :
And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel.
Gen 3:15
And the scripture, foreseeing that God would justify the heathen through faith, preached before the gospel unto Abraham, saying, In thee shall all nations be blessed.
Galatians 3:8
Following out from Genesis, through Israel, all the types and shadows declare with more specificity the outworking of the gospel in the cross (e.g. the Passover). But it was taught in Abraham and even foreshadowed to Eve.