Sahih Bukhari
Ablutions (Wudu)
Volume: 001 / Book Number: 004 / Hadith Number: 221
Narrated Anas bin Malik:
A Bedouin came and passed urine in one corner of the mosque. The people shouted at him but the Prophet stopped them till he finished urinating. The Prophet ordered them to spill a bucket of water over that place and they did so.