In the Book of Genesis, the Flood narrative begins in Genesis 6 with God telling Noah to build the Ark.
Genesis 6 introduces the corruption and violence that precede the Flood and presents Noah as a righteous man who finds favor with God. God then announces the coming destruction by flood and gives Noah instructions for constructing the Ark, including its material, dimensions, decks, and waterproofing.
The main narrative continues through Genesis 7, which describes the Flood and the entry of Noah’s family and the animals; Genesis 8, which describes the recession of the waters and the Ark’s landing; and Genesis 9, which describes the covenant and rainbow. This makes Genesis 6–9 the central Flood section.
Genesis 3 is the earlier account of Adam and Eve, while Genesis 12 begins the story of Abram. Genesis 6 is therefore the specific chapter to cite for the Ark’s commission, not merely the chapter where the Flood reaches its climax.