Sarah promised a son

Ge 18:9: And they said unto him, Where is Sarah thy wife? And he said, Behold, in the tent.
Ge 18:10: And he said, I will certainly return unto thee according to the time of life; and, lo, Sarah thy wife shall have a son. And Sarah heard it in the tent door, which was behind him.
Ge 18:11: Now Abraham and Sarah were old and well stricken in age; and it ceased to be with Sarah after the manner of women.
Ge 18:12: Therefore Sarah laughed within herself, saying, After I am waxed old shall I have pleasure, my lord being old also?
Ge 18:13: And the LORD said unto Abraham, Wherefore did Sarah laugh, saying, Shall I of a surety bear a child, which am old?
Ge 18:14: Is any thing too hard for the LORD? At the time appointed I will return unto thee, according to the time of life, and Sarah shall have a son.
Ge 18:15: Then Sarah denied, saying, I laughed not; for she was afraid. And he said, Nay; but thou didst laugh.