Musannaf Ibn Abi Shayba 3:5782

Ibn Idris narrated to us, from Ibn Jurayj, from Ibn Abi ‘Ammar, from ‘Abdullah ibn Babah, from Ya‘la ibn Umayyah, who said: I asked ‘Umar ibn al-Khattab. I said: "‘There is no blame upon you for shortening the prayer if you fear that those who disbelieve may afflict you’ (4:101), yet the people are now secure." He said: "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.'"