13 The LORD said to Abraham, "Why did Sarah laugh and say, 'Shall I indeed bear a child, now that I am old?'
14 Is anything too hard for the LORD? At the appointed time I will return to you, about this time next year, and Sarah shall have a son."
Abraham is visited by three "men" though one or all are manifestations of God in human form. He greets them as honoured guests, which indicates that he knows of their holy origin. God, as one of the men, reminds Abraham of his promise to grant him a child (through which Abraham will become the father of multitudes).
Sarah, Abraham's wife overhears and laughs at this because she is too old to have children. God overhears and comments as above, that nothing is too hard for God.
Though God has made this promise, he has taken some time to actually enact the promise and give Abraham a son. The nature of faith is that God will give enough for us to trust him, though it might not be enough for everybody in the same situation. Abraham was able to trust despite years until fulfilment, but Sarah was not - at least in this case.