Al-Kāfi
Hadith 935سَهْلُ بْنُ زِيَادٍ عَنْ بَعْضِ أَصْحَابِنَا أَنَّهُ كَرِهَ الْكُلْيَتَيْنِ وَ قَالَ إِنَّمَا هُمَا مَجْمَعُ الْبَوْلِ .
6. Sahl ibn Ziyad has narrated from certain persons of our people who have said the following: “He (the Imam) (a.s.), disliked the kidneys and said that they are the collectors of urine.’”