Muwatta Imam Malik Book of Jihad, Hadith 6

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).

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ زَيْدِ بْنِ أَسْلَمَ، قَالَ كَتَبَ أَبُو عُبَيْدَةَ بْنُ الْجَرَّاحِ إِلَى عُمَرَ بْنِ الْخَطَّابِ يَذْكُرُ لَهُ جُمُوعًا مِنَ الرُّومِ وَمَا يَتَخَوَّفُ مِنْهُمْ فَكَتَبَ إِلَيْهِ عُمَرُ بْنُ الْخَطَّابِ أَمَّا بَعْدُ فَإِنَّهُ مَهْمَا يَنْزِلْ بِعَبْدٍ مُؤْمِنٍ مِنْ مُنْزَلِ شِدَّةٍ يَجْعَلِ اللَّهُ بَعْدَهُ فَرَجًا وَإِنَّهُ لَنْ يَغْلِبَ عُسْرٌ يُسْرَيْنِ وَأَنَّ اللَّهَ تَعَالَى يَقُولُ فِي كِتَابِهِ {يَا أَيُّهَا الَّذِينَ آمَنُوا اصْبِرُوا وَصَابِرُوا وَرَابِطُوا وَاتَّقُوا اللَّهَ لَعَلَّكُمْ تُفْلِحُونَ }