Musnad Ahmad 30:137

Sufyan narrated to us, from al-Zuhri, from Sa’id, from Abu Hurayrah, who said: A Bedouin entered the mosque and prayed two rak'ahs, then said: "O Allah, have mercy on me and on Muhammad, and do not have mercy on anyone else with us." The Prophet, peace and blessings of Allah be upon him, turned to him and said: "You have restricted something vast." Then, it was not long before he urinated in the mosque. The people rushed toward him, but the Messenger of Allah, peace and blessings of Allah be upon him, said to them: "You were only sent to make things easy, and you were not sent to make things difficult. Pour a bucket of water, or a pail of water, over it."