Ezekiel 38:23
וְהִתְגַּדִּלְתִּי וְהִתְקַדִּשְׁתִּי וְנוֹדַעְתִּי לְעֵינֵי גּוֹיִם רַבִּים וְיָדְעוּ כִּי אֲנִי ה'
I will magnify Myself and sanctify Myself, and I will make Myself known in the eyes of many nations and they will know that I am Hashem.
Following the preliminary morning service, before pesukei d'zimrah (the verses of praise), kaddish is recited. The opening words, "Yisgadal v'yiskadash" ("Magnified and sanctified...") are based on this verse from the prophecy of Yechezkel (Ezekiel). Rav Yosef Kara* (Mahari Kara, d. circa 1130) writes that the way in which G-d will magnify and sanctify Himself is through the actions that He will take during the future war of Gog and Magog. When the nations of the world witness these wondrous deeds, they will recognize that He alone is G-d, resulting in a magnification and sanctification of His Name.
*Not to be confused with Rav Yosef Karo, the author of the Shulchan Aruch