The evidence that urine is impure and the obligation to take precautions concerning it Sahih Muslim 292b
Arabic hadith text
حَدَّثَنِيهِ أَحْمَدُ بْنُ يُوسُفَ الأَزْدِيُّ، حَدَّثَنَا مُعَلَّى بْنُ أَسَدٍ، حَدَّثَنَا عَبْدُ الْوَاحِدِ، عَنْ سُلَيْمَانَ الأَعْمَشِ، بِهَذَا الإِسْنَادِ غَيْرَ أَنَّهُ قَالَ " وَكَانَ الآخَرُ لاَ يَسْتَنْزِهُ عَنِ الْبَوْلِ أَوْ مِنَ الْبَوْلِ
Translation
This hadith is transmitted from A'mash by Abmad b. Yusuf al-Azdi, Mu'alla b. Asad, Abd al-Wahid, Sulaiman with the same chain of transmitters but for the words
" The other did not keep himself safe from being defiled by urine."