Maliks Muwatta Book 21, Hadith Number 6.
Section : Stimulation of Desire for Jihad.
Yahya related to me from Malik that Zayd ibn Aslam had said that Ubayda ibn al-Jarrah had written to Umar ibn al-Khattab mentioning to him a great array of Byzantine troops and the anxiety they were causing him. Umar ibn al-Khattab wrote in reply to him, “Whatever hardship befalls a believing slave, Allah will make an opening for him after it, and a hardship will not overcome two eases. Allah the Exalted says in His Book, ‘O you who trust, be patient, and vie in patience; be steadfast and fear Allah, perhaps you will profit.'” (Sura 3 ayat 200).