Imam al-Shafi'i (may Allah have mercy on him) says: The Prophet (peace and blessings be upon him) has guided us that "shortening the prayer while travelling without fear is a charity from Allah"; shortening the prayer is not obligatory, and shortening while travelling without fear may be done if the traveller wishes. Sayyidah Aishah (may Allah be pleased with her) says that the Messenger of Allah (peace and blessings be upon him) prayed both shortened and complete prayers while travelling.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5421