Musnad Ahmad 2:93

Ibn Idris narrated to us, Ibn Jurayj informed us, from Ibn Abi Ammar, from Abdullah ibn Babiyah, from Ya'la ibn Umayyah, who said: I asked Umar ibn al-Khattab, saying: "{There is no blame upon you for shortening the prayer if you fear that those who disbelieve may disrupt you} [An-Nisa: 101], yet the people are now secure?" Umar said to me: "I wondered about what you wondered about, so I asked the Messenger of Allah (peace and blessings of Allah be upon him) about that. He said: 'It is a charity which Allah has bestowed upon you, so accept His charity.'"