Sahih Bukhari Volume 9, Book 93, Hadith Number 523.

Narrated By Ibn ‘Abbas : The Prophet used to say at the time of difficulty, ‘La ilaha il-lallah Al-‘Alimul-Halim. La-ilaha il-lallah Rabul- Arsh-al-Azim, La ilaha-il-lallah Rabus-Samawati Rab-ul-Ard; wa Rab-ul-Arsh Al-Karim.’ (See Hadith No. 356 and 357, Vol. 8)