בראשית טו,ה: וַיוֹצֵּא אֹתוֹ הַחוּצָה וַיֹאמֶר הַבֶט -נָא הַשָמַיְמָה וּסְפֹר הַכּוֹכָבִים אִם-תוּכַל לִסְפֹר אֹתָם וַיֹאמֶר לוֹ כֹּה יִהְיֶה זַרְ עֶךָ:
The Baal haTurim points out that Gematriah of "koh" is twenty-five. G-d was hinting to Avroham that Yitzchak would be born in twenty-five years. Avrohom was seventy-five years at the B'ris bein ha'Besarim (where this Pasuk was said), and Sarah gave birth to Yitzchak when he was a hundred.
Moreover, the Gematriyah of "yih'yeh" is thirty, a hint that in every generation there will be thirty Tzadikim of the caliber of Avroham Avinu that would sustain the world. The Gemara (Chullin 92a) additional states that there will always be 30 righteous non-Jews whose merits will sustain the non-Jewish nations of the world. This is understandable since Avroham was the father of nations, Jewish and non-Jewish.
