Musnad Ahmad 2:159

It was narrated that Ya`la bin Umayyah said:

I said to `Umar bin al-Khattab. People are shortening the prayer today, and Allah says: “If you fear that the disbelievers may put you in trial (attack you)` [an-Nisa` 4:101). But that time [i.e., time of fear] has gone, He said: I wondered the same thing as you are wondering, and I asked the Messenger of Allah (ﷺ) about that. He said: `It is a charity that Allah has bestowed upon you, so accept His charity.” `Abdur-Razzaq told us, Ibn Juraij told us I heard `Abdur-Rahman bin ‘Abdullah bin Abi ‘Ammar narrate... and he mentioned [the same report].