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): "Indeed, Allah Almighty has said: 'Shorten the prayers if you fear that the disbelievers may harm you' [النساء: 101] but now people are in safety?" So Umar (may Allah be pleased with him) said: "I was also surprised at what you are surprised at, so I asked the Messenger of Allah (ﷺ), and he (ﷺ) said: 'It is a charity, which Allah has given to you, so accept His charity.'"
Hadith Referenceمسند الإمام الشافعي / كتاب الصلاة / 354
Hadith Takhrijأخرجه مسلم: صلاة المسافرين، باب صلاة المسافرين وقصرها: 686.