Mishkat al-Masabih 1335
Ya'la b. Umayya said:
I remarked to ‘Umar b. al-Khattab that God had said, “You may shorten the prayer,” only “if you fear those who are infidels may afflict you” (Al-Qur’an; 4:101), whereas the people were now safe. He replied that he had wondered about the same matter, so he asked God's Messenger and received the reply, “It is an act of charity which God has done to you, so accept His charity.” 

Muslim transmitted it.
وَعَن يعلى بن أُميَّة قَالَ: قلت لعمر بن الْخطاب: إِنَّمَا قَالَ اللَّهُ تَعَالَى (أَنْ تَقْصُرُوا مِنَ الصَّلَاةِ إِنْ خِفْتُمْ أَنْ يَفْتِنَكُمُ الَّذِينَ كَفَرُوا)
فَقَدْ أَمِنَ النَّاسُ. قَالَ عُمَرُ: عَجِبْتُ مِمَّا عَجِبْتَ مِنْهُ فَسَأَلْتُ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ. فَقَالَ: «صَدَقَةٌ تَصَدَّقَ اللَّهُ بِهَا عَلَيْكُمْ فَاقْبَلُوا صدقته» رَوَاهُ مُسلم