(a) Ya'la bin Umayyah said: I said to Sayyiduna Umar bin al-Khattab (may Allah be pleased with him): "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], and the people are now safe. He said: I wondered at the same thing that you wonder at, so I asked the Prophet (peace and blessings be upon him), and he said: "It is a charity that Allah has bestowed upon you, so accept His charity."
(b) In the narration of Abu Asim, he (Ya'la) said: I said to Sayyiduna Umar bin al-Khattab (may Allah be pleased with him): Allah Almighty says: "if you shorten the prayer, if you fear that those who disbelieve may put you to trial" [Surah al-Nisa: 101]; and at the end of it he said: "It is Allah's charity upon you, so accept it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5378