It is narrated from Sayyiduna Ali (may Allah be pleased with him) that once we were praying with the Prophet (peace and blessings be upon him), when suddenly the Prophet (peace and blessings be upon him) left the prayer and went home, and we remained standing as we were. After a short while, the Prophet (peace and blessings be upon him) returned, and drops of water were dripping from his head. Then the Prophet (peace and blessings be upon him) led us in prayer again from the beginning, and after finishing, he said: "When I stood for prayer, I remembered that I had become impure by choice, so if any of you feels some disturbance in his stomach, or experiences a condition like mine, he should do as I did."