Allah says in the Qur’an: “And when you go forth in the land there is no sin upon you, if you shorten your prayer when you fear the disbelievers may attack you.” This concession is not limited to situations of danger.
Ya’la ibn Umaiyyah said: “I said to ‘Umar ibn al-Khattab: ‘Explain to me why the people shorten the salah when Allah says, ‘And when you go forth…[the preceding verse] and those days are gone now!’ ‘Umar said: ‘I wondered about that too and I mentioned that to the Prophet and he said: “This is a charity that Allah, the Exalted, has bestowed upon you, so accept His charity.'” This is related by the group.
At-Tabari records that Abu Munib al-Jarshi mentioned this verse to Ibn ‘Umar and said: “We are safe now and are not in fear, should we, then, shorten the salah’?” He answered him: “You have indeed in the Messenger of Allah a beautiful pattern (of conduct).”
The issue was also referred to ‘Aishah and she said: “The salah was made fard in Makkah in sets of two rak’at. When the Prophet sallallahu alehi wasallam came to Medinah, two rak’at were added to each salah except the maghrib salah because it is the witr of the daytime, and the dawn prayer due to its lengthy Qur’anic recital. But if one travels, he performs the original prayer [i.e., only two rak’at].” This is related by Ahmad, alBaihaqi, Ibn Hibban, and Ibn Khuzaimah. Its narrators are trustworthy.
Ibn al-Qayyim says: “The Prophet would pray only two rak’at for those prayers which consisted of four, whenever he travelled until he returned to Medinah. And it is not confirmed that he ever prayed four rak’at [while travelling], and none of the imams differ on this point, although they do differ about the ruling of shortening the salah.”
‘Umar, ‘Ali, Ibn Mas’ud, ibn ‘Abbas, ibn ‘Umar, Jabir and the Hanafi scholars say that it is fard. The Maliki school holds that it is sunnah mu’akadah (the stressed one); it is even more emphasized than the congregational salah. If the traveller cannot find another traveller to lead him in the salah, he may pray by himself as it is disliked that he should follow one who is a resident [i.e., and pray four rak’at] according to the Maliki school. The Hanbali school holds that it is preferred for the person to shorten the prayer rather than to pray the complete salah. The Shaf’i school has a similar opinion, if the person has travelled a sufficient distance.