Fiqh-us-Sunnah Volume 2, Supererogatory Prayer, Fiqh 2.111.

Section : Whence one may shorten one’s salah.

The majority of the scholars are of the opinion that it is permissible to shorten one’s salah when one leaves one’s residence and is outside of one’s city, and that is a condition, and he is not to resume his regular salah until he reaches the first houses of his city.

Ibn al-Munzhir says: “I do not know of the Prophet shortening his salah during any of his travels until after he had left Medinah.”

Anas relates: “I prayed four rak’at at Zhul-Halifah.” This is related by the group. Some of the early scholars say that if one makes the intention to travel, he may shorten his salah even if he is in his house.