Prayer on a Ship
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 62
وَقَالَ لَهُ إِبْرَاهِيمُ بْنُ مَيْمُونٍ: نَخْرُجُ إِلَى اَلْأَهْوَازِ فِي اَلسُّفُنِ فَنُجَمِّعُ فِيهَا اَلصَّلاَةَ فَقَالَ «نَعَمْ لَيْسَ بِهِ بَأْسٌ» فَقَالَ لَهُ فَنَسْجُدُ عَلَى مَا فِيهَا وَ عَلَى اَلْقِيرِ قَالَ «لاَ بَأْسَ.
Ibrahim ibn Maymun said to him (Abu Abdullah عليه السلام): "We travel to Ahwaz in ships, and we combine the prayers on board." He عليه السلام replied: "Yes, there is no harm in that." He further asked: "And we prostrate on what is in the ship and on the pitch (tar)?" Imam عليه السلام said: "There is no harm."
- Ṣaḥīḥ al-Faqīh, p. 66