וַיִּפֹּ֧ל אַבְרָהָ֛ם עַל־פָּנָ֖יו וַיִּצְחָ֑ק וַיֹּ֣אמֶר בְּלִבּ֗וֹ הַלְּבֶ֤ן מֵאָֽה־שָׁנָה֙ יִוָּלֵ֔ד וְאִ֨ם־שָׂרָ֔ה הֲבַת־תִּשְׁעִ֥ים שָׁנָ֖ה תֵּלֵֽד׃
Then Abraham fell upon his face, and laughed, and said in his heart: ‘Shall a child be born unto him that is a hundred years old? and shall Sarah, that is ninety years old, bear?’
Read kabbalistic (mystical) interpretation of Genesis 17:17: and classical mystical sources.