لا إلَهَ إِلا اللَّهُ الْعَظِيمُ الْحَلِيمُ ، لا إلَهَ إلا اللَّهُ رَبُّ الْعَرْشِ الْعَظِيمُ ، لا إلَهَ إلا اللَّهُ رَبُّ السَّموَاتِ وَ رَبُّ الأَرْضِ وَرَبُّ الْعَرْشِ الْكَرِيمُ
La ilaha ilaAllah al-‘Adheem al-Haleem, la ilaha ilaAllah Rabb al-Arsh al-‘adheem, la ilaha ilaAllah Rabb as-Samawat wa Rabb al-Ard wa Rabb al-Arsh al-Kareem
There is no god but Allah, the All-Powerful, the Forbearing; there is no god but Allah, Lord of the mighty Throne; there is no god but Allah, Lord of heaven, Lord of earth, and Lord of the noble Throne.
[Sahih al-Bukhari, 8/154, Muslim, 4/2092]
Ibn Abbas (ra) reported that when the Prophet (saws) felt distressed he would say (the above)
Daily Dua'a 
Why did the Prophet (saws) feel distress when saying this dua? Perhaps it filled him with fear of Allah?
Prophet Mohamed, SAAWS, went though many hardships and stressful situations, he was sworn at, he was humiliated by the mushrekeen, rocks were thrown at him, promises were broken, there were many setbacks in the journey of his message and these Duaas was his way of asking Allah SWT for help and support.