Abu Dawud Book 2, Hadith Number 1195.

Chapter : The prayer of a traveller.

Narated By Ya’la b. Umayyah : I remarked to ‘Umar b. al-Khattab: Have you seen the shortening of the prayer by the people today while Allah has said; “If you fear that those who are infidels may afflict you” (iv. 101), whereas those days are gone now? He replied: I have wondered about the same matter for which you wondered. So I mentioned this to the Apostle of Allah (PBUH). He said: It is an act of charity which Allah has done to you, so accept His charity.