The holy scriptures talk about a flood that covered the whole earth, and how Noah and his family were saved, and the animals he collected, and building an ark. After many days, Noah sent out a dove that returned with an olive twig, which gave assurance of new life in the future. God also promised that He would be at peace with mankind and not send another devastating flood. As a sign of this promise, God put a rainbow in the sky. The exact scripture can be found in Genesis 6:5 through Genesis 9:17, linked here.