Say at least three times:

سُبْحَانَ رَبِّيَ الْأَعْلَى

Subhaana Rabbiyal-A'laa

Glory is to my Lord, the Most High. This dua should be repeat again and again with the emotion of the mind. Reciting this tasbeeh at least three times is the conduct and instruction of Rasulullah (ﷺ). Most of the narrations mention "Subhaana Rabbiyal-A 'laa" and some of the hadiths describe "Subhana Rabbiyal-A 'laa wa bihamdihi". After reciting every verse of mercy, he stopped and asked Allah for it, and after reciting every verse of punishment, he stopped and asked Allah for refuge from it.

Reference: Muslim: 772