וַתַּהַר֩ וַתֵּ֨לֶד שָׂרָ֧ה לְאַבְרָהָ֛ם בֵּ֖ן לִזְקֻנָ֑יו לַמּוֹעֵ֕ד אֲשֶׁר־דִּבֶּ֥ר אֹת֖וֹ אֱלֹהִֽים׃
And Sarah conceived, and bore Abraham a son in his old age, at the set time of which God had spoken to him.
Read halakhic rulings (Jewish law) on Genesis 21:2: and classical halakhic sources.