Al-Kāfi - Volume 3 > Where to Apply Rules about Salah (Prayer) on a Journey
Hadith #2

2ـ الْحُسَيْنُ بْنُ مُحَمَّدٍ عَنْ مُعَلَّى بْنِ مُحَمَّدٍ عَنِ الْحَسَنِ بْنِ عَلِيٍّ الْوَشَّاءِ قَالَ سَمِعْتُ الرِّضَا (عَلَيْهِ الْسَّلام) يَقُولُ إِذَا زَالَتِ الشَّمْسُ وَأَنْتَ فِي الْمِصْرِ وَأَنْتَ تُرِيدُ السَّفَرَ فَأَتِمَّ فَإِذَا خَرَجْتَ بَعْدَ الزَّوَالِ قَصِّرِ الْعَصْرَ.

2. Al-Husayn ibn Muhammad has narrated from al-‘Ala’ ibn Muhammad from al-Hassan ibn Ali al-Washsha’ who has said the following: “I once heard al-Rida’, ‘Alayhi al-Salam, say, ‘At Zawal (declining of the sun toward the west at noontime) if you are in a city and want to travel, if you go out after Zawal perform al-‘Asr Salah in the reduced form.’”