It is narrated from Ya'la bin Umayyah (may Allah be pleased with him) that he said: I asked Umar bin Khattab (may Allah be pleased with him): "Allah Almighty has mentioned shortening (the prayer) in a state of fear, but where is the shortening besides fear?" Umar bin Khattab (may Allah be pleased with him) replied: "I was also surprised by the same thing that surprised you. So I asked the Messenger of Allah (peace and blessings be upon him), and he said: 'It is a charity that Allah has given to you, so accept His charity.'"
Hadith Referenceمسند الإمام الشافعي / كتاب الصلاة / 353
Hadith Takhrijأخرجه مسلم: صلاة المسافرين، باب صلاة المسافرين وقصرها: 686.