Water; Its Purity and Impurity
Man Lā Yaḥḍuruh al-Faqīh|Volume 1|Book 1|Chapter 2
وَ سَأَلَ عَلِيُّ بْنُ جَعْفَرٍ أَخَاهُ مُوسَى بْنَ جَعْفَرٍ عَلَيْهِ اَلسَّلاَمُ: عَنِ اَلْبَيْتِ يُبَالُ عَلَى ظَهْرِهِ وَ يُغْتَسَلُ مِنَ اَلْجَنَابَةِ ثُمَّ يُصِيبُهُ اَلْمَطَرُ أَ يُؤْخَذُ مِنْ مَائِهِ فَيُتَوَضَّأَ بِهِ لِلصَّلاَةِ فَقَالَ «إِذَا جَرَى فَلاَ بَأْسَ بِهِ.
Ali ibn Ja'far asked his brother, Imam Musa ibn Jafar Al-Kadhim عليه السلام about a roof where people urinate and perform ritual washing after major impurity (ghusl), and then it rains. He asked: "Can the water that flows from it be used for ablution (wudu) for prayer?" Imam عليه السلام said: "If it flows, there is no harm in it."
- Ṣaḥīḥ al-Faqīh