Prayer During Travel
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 59
وَ سَأَلَ مُحَمَّدُ بْنُ مُسْلِمٍ أَبَا عَبْدِ اَللَّهِ عَلَيْهِ اَلسَّلاَمُ فَقَالَ لَهُ: اَلرَّجُلُ يُرِيدُ اَلسَّفَرَ مَتَى يُقَصِّرُ قَالَ «إِذَا تَوَارَى مِنَ اَلْبُيُوتِ» قَالَ قُلْتُ لَهُ اَلرَّجُلُ يُرِيدُ اَلسَّفَرَ فَيَخْرُجُ حِينَ تَزُولُ اَلشَّمْسُ فَقَالَ «إِذَا خَرَجْتَ فَصَلِّ رَكْعَتَيْنِ.
Muhammad ibn Muslim asked Abu Abdullah عليه السلام about when a person who intends to travel should begin shortening their prayers. He عليه السلام replied: "When he is no longer within sight of the houses." Muhammad then said: "If a person intends to travel and leaves at the time of the sun's decline (midday), what should he do?" He عليه السلام replied: "When you leave, pray two units (Rak'ahs)."
- Ṣaḥīḥ al-Faqīh, p. 62