Genesis 15-17 details God's covenant with Abraham. God promises Abraham that his descendants will be as numerous as the stars and that he will possess the land of Canaan. Abraham believes God and it is credited to him as righteousness. God establishes the covenant by having Abraham sacrifice animals and pass between their halves, and promises the Israelites will be enslaved for 400 years before returning to Canaan. Sarai gives her servant Hagar to Abraham to bear a child, and Hagar conceives Ishmael. God renames Abraham and Sarai, changes Sarai's name to Sarah, and establishes circumcision as the sign of the covenant, promising Abraham many descendants and kings through Isaac despite his and Sarah's old age.