Jami` at-Tirmidhi 147

Abu Hurairah narrated:"A Bedouin entered the Masjid while the Prophet was sitting. He prayed, then when he was finished, he said: 'O Allah! Have mercy upon me and Muhammad, and do not have mercy on anyone along with us.' The Prophet turned, towards him and said: 'You have restricted something that is unrestricted.' It was not long before he was urinating in the Masjid. So the people rushed to him. But Prophet said: 'Pour a bucket of water over it - or - a tumbler of water over it.' Then he said: 'You have been sent to make things easy (for the people); you have not been sent to make things difficult for them