Genesis 17:21
But my covenant will I establish with Isaac, which Sarah shall bear unto thee at this set time in the next year.
Cross-reference
Genesis 17:19 is God's initial announcement of the promise, specifying the son will be named Isaac and the covenant will be established with him.
In Genesis 18:10, God reaffirms the same specific timing and that Sarah will bear the son, making the promise more immediate and personal.
Genesis 21:2 shows the direct fulfillment of the promise: Sarah conceives and bears a son at the appointed time God had spoken.
Genesis 21:3 records Abraham naming the son Isaac, just as God commanded in the earlier promise.
Genesis 21:10-12 shows this distinction enforced: Sarah demands Ishmael's expulsion and God affirms it, saying 'in Isaac your seed shall be called.'
Genesis 26:2-5 shows God directly reaffirming the covenant to Isaac — 'the oath which I swore to Abraham your father' — confirming Isaac as the covenant heir promised here.
Genesis 18:14 poses the question 'Is anything too hard for the LORD?' right after the promise, challenging doubt about this very birth.
In Genesis 21:12, God tells Abraham to listen to Sarah, affirming the promise will continue through Isaac, the promised child.
Genesis 6:18 establishes God's covenant with Noah and his offspring, showing a pattern of divine promise to specific descendants.
Exodus 2:24 recalls this covenant with Isaac during Israel's oppression.
In 2 Kings 4:16, Elisha promises a barren woman a son 'about this season, next year' — the same pattern of God pledging a child at an appointed time.
In Matthew 1:21, God again announces a covenant son born by His promise — Jesus as the ultimate fulfillment of the covenant line God here establishes through Isaac.