Reference : Hisn al-Muslim 139

اللَّهُمَّ لاَ سَهْلَ إِلاَّ مَا جَعَلْتَهُ سَهْلاً،
وَأَنْتَ تَجْعَلُ الْحَزْنَ إِذَا شِئْتَ سَهْلاً
Allāhumma lā saḥla illā ma ja`altahu saḥla 
wa anta taj`alu ‘l-ḥazna idhā shi'ta saḥla.


O Allah, there is no ease other than what You make easy. 
If You please You ease sorrow.

Reference:
Ibn Hibban in his Sahih (no. 2427), and Ibn As- Sunni (no. 351). Al-Hafidh 
(Ibn Hajar) said that this Hadith is authentic. It was also declared 
authentic by 'Abdul-Qadir Al-Arna'ut in his checking of An-Nawawi's 
Kitabul-Athkarp. 106.

Chapter: For one whose affairs have become difficult