Cudapanthaka (or Panthaka the Younger), called in

Chinese Kanmen Luohan, or

​"The Arhat Guarding a Door."

Cudapanthaka is famed for being slow-witted. So slow was he that he was unable to learn even a single verse, and the other disciples made fun of him for it. But the Buddha, using skillful means, taught him to sweep (in some versions, to wipe) and repeat a simple verse, such as "Sweep the floor, sweep the floor, sweep the floor…" By repeating this over and over, he learned to focus his mind, attained enlightenment, and actually became a great teacher.
Another story says he had great strength, and used to knock roughly on people's doors to beg for food. Once, he knocked on an old, rotten door, and it fell to pieces! So the Buddha gave him a ringed staff (with which he is often portrayed) and told him to pound the ground with it, instead of pounding on the door with his fist.
Being careful of doors, and sweeping carefully, gives us the sense of "watching the doors of the senses to keep things pure."