Prayer During Travel
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 59
وَ سَأَلَ عَبْدُ اَلرَّحْمَنِ بْنُ اَلْحَجَّاجِ أَبَا عَبْدِ اَللَّهِ عَلَيْهِ اَلسَّلاَمُ: عَنِ اَلرَّجُلِ يُصَلِّي اَلنَّوَافِلَ فِي اَلْأَمْصَارِ وَ هُوَ عَلَى دَابَّتِهِ حَيْثُمَا تَوَجَّهَتْ بِهِ قَالَ «لاَ بَأْسَ.
Abd al-Rahman ibn al-Hajjaj asked Imam Jafar ibn Muhammad Al-Sadiq عليه السلام about a man performing supererogatory (nafilah) prayers in towns and cities while on his mount, wherever it faces. He عليه السلام replied: "There is no harm."
- Ṣaḥīḥ al-Faqīh, p. 65