It is narrated from Sayyiduna Ya'la bin Umayyah (may Allah be pleased with him) who says: I asked Sayyiduna Umar bin Khattab (may Allah be pleased with him), "Allah Almighty has said: 'When you travel in the land, there is no sin on you if you shorten the prayer, if you fear that the disbelievers may harm you.' (Surah An-Nisa, verse number: 101) Now people are in peace, there is no fear of any enemy, so why do we still shorten the prayer?" Sayyiduna Umar (may Allah be pleased with him) replied: "I was also surprised at the same thing at which you are surprised. I asked the Messenger of Allah (peace and blessings be upon him) the same question, and he (peace and blessings be upon him) said: 'Shortening the prayer is a charity from Allah to you, so accept Allah's charity.'"
Hadith Referenceالسنن المأثورة / باب ما جاء في الصلاة في السفر / 15
Hadith Takhrijصحيح مسلم، المساجد ومواضع الصلاة باب صلاة المسافرين وقصرها، رقم : 686