Allaahumma laa sahla ‘illaa maa ja’altahu sahlan wa ‘Anta taj’alul-hazna ‘ithaa shi’ta sahlan

It means:

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

The reference is as follows:
Ibn Hibban in his Sahih (no. 2427), and Ibn As-Sunni (no. 351). Al-Hafidh (Ibn Hajar) said that this Hadith is authentic.