[4:101] And when you are going forth in the land you would not be at fault if you shorten the prayer if you fear that you may be afflicted by those who do not believe; the disbelievers are a manifest foe to you.
The shortening of prayers is the recommended practice (sunna) in travel. Although first legislated in a time of fear, it remained even without the fear, in kindness to [God's] servants. When the shortening of prayers because of traveling became obligatory (far?), they were given leave for supererogatory devotion on their camel mount as a substitution, in whichever way [the mount] turned without seeking the qibla, and likewise for the person on foot. This was so that it would be known that permission to engage in talk with God is ongoing, at any time. If you wish to take part in it, you may do so whenever you wish. If you want to stay away as permitted, you may do as you wish. This is the most extraordinary kindness and a protection of the practice (sunna) of faithfulness (wafāÌ) and the confirmation of the meaning of friendship (walāÌ).