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
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«إِذَا جَرَى فَلاَ بَأْسَ بِهِ.
English translationBab Ul Qaim Publications
Hadith.6 - Ali ibn Ja'far asked his brother, Imam Musa ibn Jafar Al-Kadhim (as) 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 (as) said: "If it flows, there is no harm in it."
Translation: Bab Ul Qaim Publications·Thaqalayn link·The Arabic above is the original.