It is narrated from Hazrat Zaid bin Aslam that Sayyiduna Abu Ubaidah bin Jarrah (may Allah be pleased with him) wrote to Sayyiduna Umar (may Allah be pleased with him) about the Roman armies and his fears. Sayyiduna Umar (may Allah be pleased with him) replied in writing: After praise and blessings, know that when a hardship befalls a believing servant, then after it Allah the Exalted grants happiness, and one hardship cannot overcome two eases, and surely Allah the Exalted says in His Book: "O you who believe! Be patient in hardships, and be steadfast against the disbelievers, and remain firm in jihad, and fear Allah, so that you may attain salvation."