Ya'la bin Munabbih (may Allah be pleased with him) says: I asked Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) about the saying of Allah Almighty: "There is no sin upon you if you shorten the prayer, if you fear that those who disbelieve may put you to trial" [Surah al-Nisa: 101]. The people are now safe, so why the shortening? Sayyiduna Umar (may Allah be pleased with him) said: I too wondered as you do, so I asked the Prophet (peace and blessings be upon him): What is this? He (peace and blessings be upon him) said: "It is a charity that Allah has bestowed upon you, so accept it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5420