Sahih Muslim Book 4, Hadith Number 1461.

Chapter : Compensation of the missed prayer and excellence of observing it promptly.

Yahya b. Umayya said: I told ‘Umar b. al-Khattab that Allah had said: “You may shorten the prayer only if you fear that those who are unbelievers may afflict you” (Qur’an, iv. 101), whereas the people are now safe. He replied: I wondered about it in the same way as you wonder about it, so I asked the Messenger of Allah (may peace be upon him) about it and he said: It is an act of charity which Allah has done to you, so accept His charity.