Musnad Abu Ya'la al-Mawsili 2:42

Ubayd Allah ibn Umar narrated to us, he said: Yahya ibn Sa'id narrated to me, from Ibn Jurayj, he said: Abd al-Rahman ibn Abd Allah ibn Abi Ammar narrated to me, from Abd Allah ibn Abihi, from Ya'la ibn Umayyah, he said: I said to Umar: "Why do people shorten the prayer today, when it has been said: 'If you fear that those who disbelieve will cause you trouble' (4:101), and that day has passed?" He said: "I wondered about what you wondered about, so I mentioned that to the Messenger of Allah (peace and blessings of Allah be upon him). He said: 'It is a charity that Allah has bestowed upon you, so accept His charity.'"