Ya'la (may Allah be pleased with him) says: I said to Umar bin al-Khattab (may Allah be pleased with him): Allah Almighty says: "(that) you shorten the prayer if you fear that (the disbelievers) may put you to trial" [Surah al-Nisa: 101]? He said: I too wondered at what you wonder at, so I asked the Messenger of Allah (peace and blessings be upon him), and 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 / 5419