Muwatta Malik Book 9, Hadith 18
Yahya related to me from Malik that he had heard that Abdullah
ibn Abbas used to shorten the prayer when he travelled a distance
equivalent to that between Makka and Ta'if, and that between Makka and
Usfan and that between Makka and Jedda.
Malik said, "That is
four mail-stages, and to me that is the most preferable distance for
shortening the prayer."
Malik said, "Someone who intends to
travel does not shorten the prayer until he has left the houses of the
village. And he does not do it in full until he comes to the first
houses of the village, or is nearby."
Yahya related to me from Malik from Nafi that he used to travel one mail-stage with Ibn Umar, and he would not shorten the prayer.
Yahya related to me from Malik from Ibn Shihab from Salim ibn Abdullah that Abdullah ibn Umar used to say, "I pray the prayerof a traveller as long as I am undecided whether to remain somewhere or not, even if I am detained for twelve nights."