From Sahih Bukhari (Hadith 6346):
Narrated Ibn `Abbas:Allah's Messenger (ﷺ) used to say at a time of distress, "La ilaha illal-lahu Rabbul-l-'arsh il-'azim, La ilaha illallahu Rabbu-s-samawati wa Rabbu-l-ard, Rabbu-l-'arsh-il-Karim
Reference: Bukhari 6346